8K has aftertouch
8K can have expanded simms for sampler
3K no sampler
Best thing to do is try both keyboards if you can get the opportunity. Iown the PSR8000 and have waited 10 years to see if Yamaha can comeup with a replacement worth the extra expense. Only the Tyros has come close and i know that the PSR 3000 is supposedly a mini tyros. I have played the PSR 3000 in the shops but have not been tempted purely because in terms of sound, the difference between the 8000 and the 3000 is not great enough to merrit the cost. The sequencer is similar. There are more DSP's with the 3000, greater polyphony 128 compared to 64 on the 8000. That can be an issue if you are into sequencing but has not been and isue in live play so far.
the 8000 is real heavy but then its built for heavy duty use . I know there are others here that love their 3000 but it feels plasticy and not the best build quality to me.
I have used the 8000 for over 10 years for church and use with the choir and my band.In ten years i have had to replace the touch sensitivity strip twice. Other than that its still going strong.
If you want something new because its new then go for the 3000
But if you can get a 8000 for under £500 the snap it up !