Louder in terms of perceived volume? Or, actual SPLs? My "guess" is they would have similar SPL but the 350 with 2 cabs would have more perceived volume due to the increased dispersion, the cabs being closer to your ears, and coupling. However, you might run out of clean headroom a little sooner on the 350.
Now, if you're trying to decide, I would always opt for the more powerful amp instead of the smaller amp and 2 cabs. You can always add a cab later on if you want. But you can't turn a 350 watt amp into an 800 watt amp. Even if you don't need the power of 800 watts, the headroom is nice and the volume is controllable enough to play the smallest rooms without overwhelming them.
Now, if you're trying to decide, I would always opt for the more powerful amp instead of the smaller amp and 2 cabs. You can always add a cab later on if you want. But you can't turn a 350 watt amp into an 800 watt amp. Even if you don't need the power of 800 watts, the headroom is nice and the volume is controllable enough to play the smallest rooms without overwhelming them.