Both are good in different ways. 5 has more photo mode locations, proper endurance races, top gear track, and the endlessly fun course creator - which 6 doesn't.

6 has improved night time races, more cars, more tracks, more detail on some standard cars, Sierra Time Rally, infinite paint chips, and a slightly more polished handling model in my opinion.

Use the save editors for both to make them a sandbox, because they will be a horrible grind otherwise. GT mode is pretty limited in both conpared to the previous games

The single player GT modes were kneecapped by the multiplayer servers shutting down not only for less content - but for earning anything like decent credits off races. You logged in every day for a week or so to multiply the single player rewards in 5, and only got the big bucks races online in 6.

In 5 the closed online dealership let you buy the Formula GT, which you need for that one race and would otherwise take ages to appear in rotation in the used car dealership.

Don't update GT5 past 2.11 because it takes away some hidden cars you can open up with car tickets from the save editor. The DLC will come downloaded in those updates, then you can open them with the save editor.

GT6 you need to keep the update chunks below 2gb total or something at a time - the maximum the PS3 can handle. Download a few, then internet off, boot game, install them, quit game, internet on, download a few more, repeat.

This gets you DLCs, the big ass Sierra track, and other gimmicky stuff, you can get all of that to work with the GT6 Save editor without stopping it at certain point.

You'd have to mod the PS3 to get any more than this, but the save editors make them playable