I was in a similar situation recently. Couldnt decide, but in the end, I thought, I'd rather spend the extra now and not worry than spend less and wind up paying for it in the long run.
Im not done with my machine yet, but I got the 1/8" steel rails mounted to the 3 foot 3060 beam and it is massive. 3030 may well have been sufficient afterall. But Im already sorta wishing I had 1/4in rails before Im even done with the machine, if I'd gotten the 3030 gantry that'd be 2 things I wasnt sold on. If you can swing it, you might as well do it now eh.