Best, of course, is the 3090. But of the two the 3080 is better. Yes, you lose a touch of RAM, but the more GPU power the better for iClone.
Honestly, unless you bump up the RAM significantly you won't gain much by gaining 2GB more but with a slower GPU. If you are THAT close to running out (IOW, if 2GB would help) then you're screwed anyway (because things start slowing down significantly even with more RAM once the scene grows in complexity).
To give a concrete example, I have a Titan with 12GB and if I get around 8GB loaded iClone slows down. A lot. Nothing really to do with RAM nearly as much as processing power (and the Titan is powerful, only around 30% slower than the 3090). I haven't tested my new machine with the 3090 card in it to see, but I fully expect it to be a lot faster regardless of how much memory I'm using.
And when you get that close you can do other things to help. But there is no way to gain more processing power in your GPU. So that's always the key figure. Yes, you want RAM, at least 8GB, but you definitely want the fastest GPU you can afford.
Alienware Aurora R16, Win 11, i9-149000KF, 3.20GHz CPU, 64GB RAM, RTX 4090 (24GB), Samsung 870 Pro 8TB, Gen3 MVNe M-2 SSD, 4TBx2, 39" Alienware Widescreen Monitor
Mike "ex-genius" Kelley