I was in exactly the same situation last month. went for a Galaxy S over the Desire as felt that the slightly (and I do mean slightly) faster processor, bigger screen and dual cameras gave it the edge.
I've had every iPhone since day one and wanted to break away from being so tied in to one manufacturer.
Galaxy S pros
SWYPE - is really, really fast with no learning curve... it just works.
bigger screen - It really is as good as they say... stunning in fact
front camera - video calls over 3G, H & Wifi (very limited apps available for this though) I've used FRING to connect to other android and Apple 3G & 3GS. Full Skype Android video client promised for release this year.
Tethering built in (If your carrier allows, you need to check this)
GPS - no probs with UK models, so probably carrier specific, i.e. sotware issues in the USA and can be resolved. Typically 10-20 Secs to lock on to Sats.
Browsing is very snappy
2.2 Froyo is already scheduled for Sept 2010 release.
Galaxy S cons
Flash - some workarounds but really not great - need 2.2 to fix this
Speed - some lag but only with certain apps hmmm!
Kies desktop sync software - I have it working no probs, but lots of people seem to have issues? It's not great, but it does what it says on the tin for me.
Battery -not really a con, as I fully expected to have to charge it every night as I did with my iPhones... what's the point of having a superb screen and turning the brightness down???
Bottom line is that they are both great devices, with little separating them. My personal experience of the Galaxy S has been superb over the past month and I think that it's only going to get better.