I have had GT5 (Academy Edition) for just over a year but had to start a new save a week before GT6 release....up to that point I had completed about 85% of it and about 300 hours of play. I played GT6 exclusively until I got 100% ( midway through January as restarted my game) and since then I have been playing both games every day (a couple of hours a day doing WRS Time Trialling and the odd RB Champ on GT6 and a couple of hours doing anything on GT5). My opinion on where GT5 is better than GT6 (therefore making it worth getting):
- Seasonal Races, Ferrari Formula 1 Races, Formula GT races, Endurance Races, more Rally Events, harder licences, more content (it's a basically complete game so it's understandable), harder Special Events (versus Missions and Coffee Breaks), insanely harder Vettel Challenges.
Just a few points that I feel are important to know when considering buying GT5. I would definitely recommend buying GT5, especially as it is a pretty cheap price now. I only use a DS3 controller so I struggle with some things but you should be a lot better given you have a wheel. Frankly, GT5 is worth the money just for the Seasonal events, special missions, Formula GT races, Ferrari Formula 1 races and the overall greater difficulty and more content (not to mention proper endurance races). I haven't mentioned Online as I don't use it but I would guess GT5's online is not very populated (it is better than GT6 from what I hear). Finally, bear in mind that the online servers for GT5 will be discontinued in May (not sure if the seasonal events will disappear or not). Oh, and a final thought:
GT6 has all cars available to buy straight away....GT5 has the Used Car Dealership (there are only about 30 dealers to buy new cars from) which shows 6 new cars after every race is entered (or licence test entered) but some cars rarely appear in it (I have been trying to get the Chapparal 2J in it for the last 3 weeks or so and it has refreshed about 100 times or so before appearing) so can be a bit frustrating.
If you do go ahead and get it, definitely check out the GT5 section of this site as I have found it invaluable for tuning sheets, recommended cars for events or just for the best way to get the UCD to refresh quickly. GT5 also still has new seasonals every week (time trials and race events alternating weekly).
Oh, as for updates when you get the game, if you buy the Academy Edition (or XL from what others have said they are the same), the update is not majorly large....from what I remember it took me about an hour or so to download the updates and I only have a 6MB connection (max download of about 800kb/sec).
I hope some of all that has convinced you even more that it is definitely worth buying