The best thing to do would be to venture into the forums for both of the phones on this site. As a person who was on the fence between these 2 phones for a while, I'll give you my take on them.
The DX:
- has a slightly larger footprint yet noticeably larger screen real estate (decide in store which feels better)
- has google as its main search and uses google maps straight out of the box
- reception and call quality are better than any other vzw phone. 3 mics do great with eliminating most background noise
- now ships with froyo 2.2
- gps works great
- has more available accessories due to being on the market longer
The SF:
- uses a SAMOLED screen vs. the DX's LCD screen. the SAMOLED screen has vibrant colorization whereas the DX is pretty close to natural colorization. it's a preference thing. however, LCD screens do provide crisper text IF you have good eyesight to even notice the difference
- has bing as its main search and uses bing maps straight out of the box (requires some work to change this, and the best method is rooting the phone)
- some areas the phone gets much worse reception than the DX, other places it gets marginally worse reception or in par with the DX. numbers don't mean much if you can still make calls just fine, though. however, it's become a gamble whether you get a phone that doesn't sound like crap either with hearing yourself or the person on the other end saying you sound like crap
- ships with 2.1, but 2.2 should roll out in the next month or 2
- like call quality, the gps is a gamble as to how well yours will work. for some it works great, others it takes quite a while to get a satellite lock, and others the gps is near useless. it's something Samsung knows about and will be fixing in the next month or two
beyond those basics, it also depends if you'll be intending to root the phone, and even then how much you're willing to do with customization. the DX has a locked bootloader that keeps us from running custom kernels, but beyond that the 2 phones can do about the same with themes, removing bloat software from verizon, and doing overclocks with the processor.
anything else, just ask :]