If you've a normal base on the mini IMO the problem (1/48) isn't so noticable.
I'd agree if you use slotta size bases or if the cast on bases raise the figure more than a coupe mm's. The visual impact with a 3mm or larger base is of figs that are just the right height.
Also, 28mm figs are alot chunkier than real proportions so they are the appropriate width for 1/48 vehicles if a tad short. Put a 28mm fig next to a 1/56 scale chair and the figure looks enormous.
If there's any chance you will be putting in a 28mm pilot, gunner or passenger then 1/48 is the way to go. If not, then I'd go with whatever is cheapest or most available. Anything from 1:56-1/48 will look just fine on the tabletop alongside your figs. New Ray helicopters are dirt cheap if you can find them and look quite good.
I'd personally avoid 1/64 scale stuff with 28mm as it looks quite tiny, but if it looks ok to you, then go ahead.