the GT6 has that apreggio function as well.
overall, i feel that you have to put in more effort to get the best out of the GT stuff as it is built for versatility; meaning, the options available is really wide and deep but you have to know what you are doing.
for example, it takes a modular approach where you can arrange the effect chain in the order you want. this feature alone can provide lots of tone options ranging from the classic to the whacky, but as said, it can also lead you deeper into nowhere if you do not bother to learn some basic sound engineering concepts.
once you understand those concepts, you will realise the GT stuff can also be used for purposes other than for processing guitar signals - i know of people who use it to process a saxophone. as you can see, using the GT stuff will provide you some insight into another area of music, which is sound engineering.
as for the korg, i have only tried the AX1500G, and i felt that while it holds its place well for most practical use (demo recording, jamming), i eventually found it limiting after a few months. often, i would found myself wanting to go deeper in EQing for a specific tone, but because it's more or less a "set, activate and go" process on the korg, there is only so much i can do. so if you are the kind who doesn't like to deal with fuss, then the korg may be enough if you like its sound.
construction quality wise, the GT stuff wins hands down. i have people sorely regretting that they accidentally walked into my GT6 and GT8. it's no joke jamming your toes into these!
the biggest difference between the GT6 and the GT8 is that the 8 can do the following:
- dual amp sim (running two amp sims simultaneously)
- certain effects can be doubled (eg. two flangers in the chain)
- control your actual guitar amp channels (switch clean channel to another)
- compressor is a separate effect (with the 6, if you use certain effects, the compressor will not be available)
- external effect loop is a separate function in the chain (with the 6, when you use the external effect loop, you will lose the function of the distortion effects. you can still use the amp sim's distortion though.)
there are other improvements in the GT8, like a solo function which really helps your guitar sound to sing through the mix when activated. however, i believe it's a matter of EQing if you want to achieve the same thing on the GT6. the choice and features of the delays and reverbs in the GT8 are wider than the GT6 as well.
i would say that if you cannot afford the GT8, the GT6 is really sufficient to keep you really busy and curious for a long while, especially if you decide to plunge into the learning curve of fully utilising the GT6. maybe by the time you want to move on, the next GT would already arrive and you would be in a better position to buy that instead of moving to the GT8.