Start with your standard coil and let the results be your guide. If you're digging a lot of surface items, a large coil may not help until the shallow stuff is dug and cleared out, as the chances of hitting multiple, masking targets are greater with a larger coil. If targets are sparse, but you're certain something should be there, by all means get the larger coil. If finds are rich enough with a smaller coil, but peter out, you may gain a fresh advantage by going to a larger coil and by then you'll have an idea if the area is worth the expense of a new coil.
Go slow and overlap your sweeps by 50% using your existing coil, and you will in effect be giving yourself much of the depth advantage of the larger coil, though at the expense of less actual ground coverage.
OTOH, any excuse for new gear is worthy of consideration!
-Ed