First of all I would like to suggest you not to give so much weight to the brandname, because all of them are series 60 which means that their user interface as well as most of the apps are almost identical. The only real reason to see the brand is for the support.
Nokia: Surely the biggest one in selling mobile phones but support is not always perfect (most of the times it depends where you live). Not to mention that you won't get free firmware updates through Internet (you can get firmware upgrades only during the warranty period and you have to send it on a Nokia Service Center for a period of 1 hour to couple of months/as I said depending where you live).
Siemens: Well surely they have great support (at least in my country). I must admit though that past Siemens phones (especially the SL-45) were VERY BUGGY. I strongly believe though that this will not be a problem with the SX-1.And by the way it is one of the few companies that gives free firmware upgrades through Internet for all their phones.
Sendo: I don't no many things about Sendo but I have heard by many people about their very stable phones. And the free firmware upgrades through Internet are also present.
The next best thing you should look for is the features ...that feat YOUR needs.
3660: Well I can't really recomend it. Although the new keypad is very good the RAM is very limited(only 3MB free compared to more than 8 on the other s60-7650 and 3650 are not counted). I must admit though that 3650 still has the best camera, as long as the camera hardware hasn't changed on 3660,but don't expect big differences.
N-Gage: If you are a gamer go for it with no doubt. But if you just need a smartphone forget it. It also doesn't have a camera and no IrDA. It is also the cheapest one (so it's not a bad choice price-wise)
6600:Well surely it's one of the top phones but there are only few reasons to choose 6600 from the competition.
1.The great and full of brightness screen as well as
2.the fully customisable skins featured only on series60 version 2.0.
3.Series 60 ver 2.0 and the latest JAVA micro edition for future applications.
On the other side the phone is slower from the others, has incompatibilities with many series 60 programs (because of the new symbian ver 7 OS) and has many of the disadvantages present on all Nokia's series 60 smartphones like no stereo output (only the N-Gage has), no hot-swapable mmc cards,no PC connectivity other than bluetooth and IrDA (again N-Gage is not count).The package also includes a 32MB MMC card(it depends though from where you buy it).
SX-1:To say the truth I didn't like it at first but I love it now. The screen is nice but not as bright as 6600s. Hot-swapable MMC cards is also very good. Radio and stereo output give it more points. The only pain would be the keypad. But it is a lot better than that of 3650 (after all Nokia uses the same keypad layout on 7600).The package also includes the usb cable to connect it to your PC.)
X: The most featured s60 device. Good screen but again not so good looking as 6600s. It also has stereo output like SX-1(no radio tuner though). But the real advantages are
1.SD-cards compatibility which are cheaper than MMCs and in greater capacities (1GB anyone?). It also takes MMC and like the Siemens they are hot swapable.
2.Dual stereo speaker (I don't remember how the thechnology is called) making it really loud
3.28MB free to user for programs/messages/photos etc. while the SX1 has only 4MB and the 6600 6MB(ofcourse you also have the memory cards).
4.A small flash for the cmera which can make the night photos a little bit better in quality(don't expect miracles).
The only disadvantage I could see on it is the size of the keys which are very small. By the way it is the smallest s60 device.
Sendo-X is the one I recommend you to buy (although I am going for the Siemens).
Lastly I would like to recommend you to make your choice by what YOU LIKE AND NOT WHAT OTHERS SAY. Take a look on all of them and their features. See which one fits your needs best. Which looks more appealing to you. Which one is more beautifull...
Just go and see them on a shop,try them,feel them... OK I've gone too far 😃
PS:The 7700 is not an option because
1. It is not series 60 but series 90 (and there are no apps for it right now)
2. It is very big (even bigger than 9210)
3. In my opinion you must compare it with P900...