As far as reliability goes the only engine with better reliability numbers than the SBC is the Chrysler slant six. You won't have to worry about reliability with either small block Chevy.
So what is the difference. Heads. The 300 horsepower 350:
is a base 350 290 horse TBI engine pulled from the Mexican assembly line where they still make a traditional SBC for production cars and trucks. Instead of the TBI manifold it has a high rise four barrel manifold and the "hot" Chevy hydraulic EFI cam (the longest duration cam that TBI will tolerate.
The truck engine is the old 1996-2004 "Vortec" motor made before Chevy killed the SBC line in 2005. It has the best flowing SBC heads that the Chevy engineers have ever cast. They utilize raised narrower intake ports to increase the intake port size while retaining the port velocity. The biggest difference is in the "Fast Burn" combustion chambers. These heads made 55 more horsepower than the previous year's "Vortec" motor with no other changes; so they do work (raising the factory rated 350 from 200HP to 255HP). There is of course no free lunch as I will now point out.
The bad news. These heads where not designed by Chevy engineers to out flow any previous head (the earlier "Vortec head" shared the same port design), but to burn the fuel that was in the chamber better. They were not looking for more power, but lower emissions. So these are not true high performance heads (Chevy sold those as their "Bow Tie" heads over the service department counter. They do beat or match most after market heads in flow numbers up to 0.450" inch of valve lift. But it isn't about just the valve lift for as stated they burn the fuel much more completely in the combustion chamber. So if 0.450" is all the valve lift you have from your cam, then these are the heads for you.
However if you want a more aggressive cam that lifts the valves higher, you will hit the limit of these heads. Unless you first modify (machine) the heads by cutting down the valve stem oil seal bosses so that the valve spring retainer doesn't hit it at 0.470" of valve lift. In addition to cutting down the cast iron boss it will require some aftermarket valve stem oil seals. The Vortec head also uses self centering stamped steel rocker arms that also relies upon a narrow push rod guide slot in the head to keep the push rod centered. This factory machined slot must be machined (elongated) to clear a higher lift cam. Since the stock springs are near coil bind at 0.450 inches of lift different valve springs are also required. If this is beginning to bother you (having the heads machined for about the same price as the cutting tools that would allow you to do it yourself at home) another economical choice for a SBC would be Dart's Sportsman II heads, which can match the Vortec's head in low lift flow and still have clearance for a valve lift of 0.700 inches in as cast form. But that isn't part of this comparison; just pointing out how heads make the motor.
Otherwise as stated if you have a cam that has only 0.450 inches of lift these are the heads for you. All you need to do is to look in most cam grinders catalogs and the majority of cams that they have listed fit this description (0.450 inches of valve lift). That is because when these cars where new back in the sixties and seventies only a racer had a cam so aggressive as to exceed 0.450" of lift. So there is a large percentage of SBC cams that are around this lift (keep in mind push rods bend when you hit the limit of 0.470" with the Vortec unmachined; and even machined you are limited to a maximum of 0.520 of lift with Vortec heads)