I had a 10k Sherline spindle for a while and found it got very hot with prolonged use at 10k (even when I loosened the bearing preload). I suggest you test for that before you make your decision. BTW my Taig spindle works fine at high speed but that is probably irrevelant to you.
Just reread your post. You will probably want to keep your Sherline for milling aluminum at lower rpms than possible with the VFD spindle. Stainless steel is very hard to mill and most routers are not rigid enough for that.