First off, let me qualify this. I'm not writing the definitive guide to every feature on the N97 and every feature on the Milestone and then comparing them, nope, I'm writing my accounts of using both devices and how the functionality that I personally need, works in the real world on my network (Vodafone UK).
Form factor: The N97 was a reasonably pockeable device for me. My only complaint was that it could have been a tad thinner. The way the keyboard opens has a certain novelty and I liked the rubber damped thud as it snapped shut. Over time I didn't really enjoy having the screen and keyboard offset at a 45 degree angle; I think it would have been easier to type if the keys were at the same angle as the screen. The Milestone is made of metal and has a soft touch coating on the rear of the device to aid grip in the hand. There is a metal battery cover and no creaking. Anywhere. The whole thing feels solid like an industrial tool, something that has the potential to outlast it's owner. The volume rocker is a bit loose feeling, but that is apparently just the way Motorola design them, and when I shake the phone, nothing rattles, which is always a good sign. One thing that I missed from the N97 straight away was the unlock slider! The Milestone equivalent is the power button on the top right and it takes a little practice to be able to pick up the phone and unlock quickly with a single hand. The Milestone is thinner than the N97 and a little heavier. On balance I prefer the Milestone on the physical front.
Keyboard: The first time I slid the Milestone open I thought "uh oh, that looks too flat to be useful"... but I was wrong. The Milestone keyboard is strangely much easier to use than the N97. The keys seem easier to hit and give more consistent feedback when you've pressed one. Also they have included web friendly keys like "@" and "/" that don't need to be shifted. Neither keyboard is for touch typing, but the Milestone's is easier to type on in my experience - after a week of usage I am faster than I was on the N97 after 6 months.
Screen: The capacitive 854 x 480 pixel display of the Milestone knocks the N97 into a cocked hat. It responds better to touches and you can read many websites zoomed out. For any application where on screen detail is helpful (like Maps) you feel the benefit of having almost twice the number of pixels on screen.
Performance / stability: The Milestone has a faster processor than the N97 - 434Mhz vs 550Mhz - and a graphics co-processor + double the amount of RAM so it's no surprise it feels snappier most of the time. What is a surprise to me, is that so far, one week into Milestone ownership, I have not had a crash - of any kind. I have not needed to kill an application or had to do a battery pull because my phone crashed after taking a call. Nope. Just the odd slowdown as something was closed when I opened a new app (and opening apps is much quicker). Most of the time if you only have 4 or 5 apps running, opening a 6th will be instantaneous compared to the N97. Websites load faster, the built in gmail app is fast and very functional and the notification bar + multi-colour LED is such a great way to indicate you have new messages or calendar appointments, etc. you feel like someone has designed the OS based on real use, not marketing labs.
Battery life: By the time I got the v20 update, my N97 was giving stellar battery life. Not since I'd had a pre-3G S60 phone had I seen anything that could last beyond 2 days, but my N97 was able to do that with low to average daily use. The Milestone has a 1400mAh battery, but was never going to be able to equal the feats of efficiency pulled off by the N97. However, the Milestone battery life has left me impressed. I've heard all sorts of bad things about Android battery life, but with normal usage (syncing gmail & calendar all the time, a few calls + texts, an hour or so of music playing and maybe an hour of browsing via WiFi) I'm getting 1.5-2 days between charges. The N97 trumps the Milestone here though.
Apps: I am shocked by the high quality and low price of many android apps. I have got pretty much everything I need for the phone by downloading free apps from Android market, with only a few purchases made. There seems to be much more open access to the phone hardware so apps that replace the on-screen keyboard or change the way the LED notification light flashes are readily available. And widgets? Yes, proper widgets that update quietly in the background and don't hang or crash.
I'm a user of Gmail and Google Calendar, so having a built-in client that syncs with both and is snappy (especially in the case of mail) is a huge plus. SMS "conversations" with each user are shown in the default messaging app and as with the N97 some contacts can be marked as favourites and dropped onto the home screen for fast access. It's the little things that really score when you notice them, and seeing that from a contact you can click an address and Google Maps will then parse the address and show the location on a map is a great relief after the lack of such integration on the N97.
Camera: A clear win for the N97 here. The Milestone camera, while also being 5MP with a dual LED flash, is no match for the N97 in quality or speed. There is also an auto-focus bug on the Milestone that means that the auto-focus ceases to function for 24.5 days (due to a rounding error!) and then works again for the next 24.5! This is due to be fixed in the first firmware update - it has already been fixed on the Milestone's cousin, the Droid - but even with a fully functioning autofocus, the N97 has the edge. One thing worthy of mention is the number of Android apps that make use of the camera. Things like Evernote, ShopSavvy and Google Googles, show how you can use the camera for more than just snaps if you stop to think about it for a moment. I guess this is the advantage of not having a lens cover (other than not scratching the lens), apps can fire up the camera when they like without needing the user to open the cover.
Price: It's a draw as both handsets are �449 SIM free.
Conclusion: Would I return to an N97 or even Symbian? No, not in it's current form. After 7 years of using Symbian based phones, it's time for me to move on. I could be tempted back in 6 months to a year if it gets the UI overhaul and better hardware, but I fear that the app deficit is only going to get worse between then and now.