I'm going to say get the DFGT if you only play GT5, the H-shifter and clutch from the G27 are really, really badly implemented in GT5 (and only GT5) so that if you're not careful with the gas when you're shifting you'll end up in neutral, and even if you never mis-shift you're still a lot slower with manual shifting, though it is admittedly a lot of fun... But whenever I'm trying to win, I use the paddles anyway.
The DFGT and G27 are mechanically very similar, both use helical gears (the G25 and DFP used to use straight gears, which were very loud, the new wheels are both very quiet for gear-driven feedback wheels) and really, apart from the larger, leather-wrapped wheel of the G27 (and the extra pedal and shifter), there's not a huge amount of difference between the two.
Oh and another thing, if you're playing GT5 then it's safe to assume you're not sitting at a table or desk, so where would you mount the shifter? If it's just the wheel and pedals it's quite easy to come up with something halfway decent to hold it still, but unless you build a rig it'll be tricky to accommodate the shifter as well. Oh and let's not forget that the G27 only has six buttons under your thumbs, the DFGT has everything on the wheel itself which is really much better, I would've liked the option to have more buttons on my wheel but I can understand why Logitech said no.
All in all, the DFGT is perfect as a first wheel, for GT5, for people without a cockpit or the money/patience/desire/space to build one and therefore I'd recommend it over the G27.