As a price point for those thinking it's not that big a difference.
Assuming say 125' run. <3% voltage drop. Meeting Table 310.16 Allowable Ampacities. Copper wire. (Aluminum can be used but requires larger wire)
For 30 amp you could get away with direct bury UF-B cable, though I'd still put it in conduit, the UF-B in 6/2 would cost about $155.
For 50 amp you'd end up needing to use 4 gauge cable USE-2. You'd need 4-4 cable equivalent, since you need an extra conductor, and would cost around $415. You could save a small amount by using a smaller ground, which is allowed if installed properly. This is 4 separate cables though so will require a little more labor.
That's a $260 difference.
But a $260 diff vs doing it twice!?
If you can square these numbers with the electrician, buy the wire yourself, rent a trencher, you could probably save some money having him make the final connections.
USE-2 and UF-B can both be direct buried but I'd put them in conduit. <$100 in materials. The 4 gauge will be fun pulling so go with a larger conduit(1-1/2 for more pulling room, 30 degree bends to make it easier).
Don't forget a permit! Hate to have to rip it all out!
Don't forget 811 call before digging.