Depends on what you're looking for. For a daily driver a 302 or 302 derivative is the way to go. The bottom line on this one is that the Fox-bodied Mustangs blew the 5-liter market wide open and made the entire economy of scale possible for this block. 302 parts are now just as cheap as 350 parts and for Blue Oval guys there simply isn't a cheaper engine available. Add to that the fact that they have been SO experimented with that there are literally *dozens* of proven power combos available that I honestly think you'd be a fool to not build or buy one. The 351 is a great engine, but it simply hasn't benefitted from the same level of development, simple as that. On the other hand they've both been around a long time and there's no shortage of good combos for either, it's just that 5.0 parts are so much cheaper these days.
Now if you're looking for something for a tow-rig then, sure, I can get behind a solidly built 351 because the extra cubes will help with the towing torque but I have trouble justifying the milage penalty in a daily driver that doesn't tow very often. But, hey, that's just me.
For a part-time trail rig, weekend toy or competition truck I would have to go hands down with the 460. You can easily build a 400hp "basic" engine or anything up to & over 800hp if you want. The block will support just about anything you want to do with it. Granted parts aren't cheap but they don't cost what Hemi-stuff does. I've built several across the spectrum from mild to wild and not been disappointed by any of them. The 460 has loads of low-end torque and it has the power to break all but the strongest of drivetrains. A 460, NP205 & Dana 60's is an indestructable & unbeatable combination. I've built them and a combo like that will keep turning the tires and it will ALWAYS move you either forward or down but you won't stop moving until the framerails bottom out. I've run them on the road & in competition and that combo ALWAYS delivers. It may not be cheap but you always get you're money's worth and you enjoy every moment. For what it's worth I think it's the best combo available and anyone looking to build a truck just to go out & have FUN with should consider it.