I have both X10 running 1.6 and Desire on 2.2.
The battery on the X10 is HORRIBLE. I unplug it at 8 am. Read 5-10 email, send 2-5. And the battery is DEAD at 5 pm. I dont do txting on it, dont do browsing, no phone calls... It's a phone that I use exclusivly for email. In the other hand: the Desire. read 5-20 email a day, send 1-3. 1-3 phone calls, 100-250 txt a day. a lot of browsing(I pull around 100mb a day), wifi is active all day long, bluetooth too. And at 9pm it still remain 15-30%...
I heard similar spec about the Galaxy.
Only for the battery, dont buy an X10. And I agree with Chris... I dont think the X10 will ever see Froyo.
Compared to my Desire(I never tried the Captivate for that), the GPS is very unprecise... on the highway it tell me that i'm on a secondary road around 100m away. The Desire is just fine again.
i dont have personal experience with the Galaxy, but I'll never recommend a X10