Depends. Does 'no online' include the ability to get updates or not? They both have great updates that are rather different. Regardless, it's still kind of complex.
Graphically, GT6 is mostly better, although the shadows are a bit dodgy, and the framerate stutters the tiniest bit when taking some corners. It also has a better physics system, since a lot of it was overhauled (though the system in GT5 is by no means bad in any way). GT6 also has more cars, including the DLC cars from GT5, and in total it amounts to about 500 or so premium cars (with the rest being standard). In case you don't know the difference, premium cars are meticulously crafted digital renderings of cars with cockpit views and more aesthetic modification options like wings and such. Standard cars have a blacked out cockpit view, and less high quality models. Other than that though, they're the same. I'd say GT6 has around 250-300 more cars than 5, with all of them being premium. There area small handful of cars and tracks in GT5 that aren't in GT6, the cars being axed due to licensing issues (as well as the Top Gear Test Track being taken out for the same reason).
GT5 probably has more stuff to do offline, with a giant racing career complete with endurance races, many more license tests to do, a track creator, more photo locations to take pictures of your cars at, and a boatload of special events. It also has B-spec mode where you train other drivers, and sort of 'coach' them through events. Some people like this, and some people don't. You don't have a ton of control, and there's no way to speed races up, so it can be time consuming and frustrating at times.
GT6 has it's own section of special events, license tests, and a few photo locations (you can also take pictures from races in either game). It also has a leg up with more aesthetic customization options overall, and some better design in this area as well. In GT5, you could change the horn or the color of your car/rims. But once you used a paint chip or a horn, it was taken from you. In GT6, you can't change horns, but your paint chips are infinitely reusable.
GT6 has more tracks to night drive/drive in rain on too. And it's constantly getting free updates. In all the time the game has been out, there has probably been around 50 cars added in, 3-5 tracks (some with multiple layouts), several types of special events, new customization options, etc. And so far, all of it has been free. I honestly think it'll continue to be that way for 90% -if not all- of the content released for GT6. It'll definitely continue to grow and expand, meaning you're guaranteed to get more game than what you're initially paying for.
Honestly, both games are great single player, and offer tons of content. At this point, GT5 should only be like $5 or $6, so there's little reason not to just pick up both.