First of all: do you want to switch cell companies? Because you stated you're looking for a GSM version, and the 650 is currently only available in CDMA. I figure that a GSM model is coming, but there aren't any guarantees as to when or with which carriers.
I've been using the 600 since it came out last year (the CDMA version with Sprint PCS). I looked at the 650 when I first heard about it, and it does look nice. However, I didn't feel for my needs that it was worth the cost of buying a new phone at premium prices - it's more of an incremental upgrade.
Having said that, there are some special considerations for other users.
* Do you frequently forget to recharge your phone? The 650 has non-volatile memory and a removable, replacable battery. If it goes dead, you don't lose your data, and you can swap out the battery for a spare. The 600 has a good battery life, but I am a frequent charger, while some people (like my wife!) often forget and have a dead phone all too often. If the 600 goes dead, you lose any data inside. You can re-sync, but any new data entered between syncs will be in data heaven...
* Do you really, really like Bluetooth gadgets, like wireless headsets? The 600 has no Bluetooth capabilities, while the 650 does. Caveat: some carriers restrict the Bluetooth capabilities to disallow using your phone as a modem for your computer or to swap your pictures out to your computer. They'd prefer you used their more costly hardware and data services for those functions. (Sprint PCS was pretty steamed when PDAnet software came out for the 600 - users could use their phone as a WAN cellular adapter for a computer and pay nothing extra for unlimited use, while Sprint sells cellular PC-card modems for beaucoup bux and charges $80-90 for unlimited service!)
* Are you big on snapping pictures with your phone and sending them to people? The 600's camera is inferior to the 650's - but my workaround is to carry a slim megapixel camera with zoom and flash that uses SD/MMC cards (superior images to BOTH phones)! I slip the card into my phone and attach photos to emails. However, this does mean carrying ANOTHER gadget, something that some people don't want to do.
If none of those things mean enough to you to justify spending an extra $200-300 for a 650, then just get the 600 and you'll be happy. Use the extra cash to invest in a large memory card for the memory slot and get MP3 software and a minijack adapter instead!
The minijack adapter is FANTASTIC! In my car, I slip a cassette adapter into my tape deck and place the 600 in a universal dashboard mount. I can play MP3s from the phone in my stereo - but better, when I get calls, the ring tone and the incoming audio ALSO plays through the car stereo! Best damn speakerphone you'll ever hear! The touchscreen's onscreen buttons are easy to use while driving, too - if you keep it mounted close to the steering wheel and in easy viewing range.
And if by next year you decide you really wanted that 650, it'll probably be marked down to what a 600 costs now because the 700 just came out... but then you can just wait for the 750... or the 800...
Also: since it appears this is your first smartphone - LOOK CAREFULLY at the cost of data services. Some carriers (like Sprint) have cheap flat-rate unlimited-use plans, some (like AT&T) have costlier flat-rate plans, and some charge per megabyte of data - a very costly way to get Internet service for a phone that's so data-friendly.