I've got the same tank, very low tech, with a single 600 in the back corner. Getting the flow somewhat right in these corner tanks is a pain.
If you can stomach it, go with 2 x 350. Otherwise, a single 600 in a lightly planted / stocked seems to work.
Tank probably won't be lightly stocked, AQAdvisor showing my tank as ~100% stocked with what I'd like to put in there. It will be heavily planted with CO2 but none of the plants will be "high" light plants or overly demanding fertilization-wise (probably will fertilize PPS-Pro for medium light plants and see how that goes before bumping up to "high").
So, filtration does seem important here. I like the pro4s because of the grey tubing but I do have other questions. Is the range extender function good for controlling flow or does it just bypass the fine filter mesh and maintain constant flow? Do you have to use eheim's filter mesh or is fashioning your own an option?
2 350s would give me about 10x turnover (~550 g/hr) whereas 2 250s would give 9x (501 g/hr), and the single 600 would give only 6x (330 g/hr). Guess there's not much difference between the 250 and 350 function wise. Although, given how much I'm spending elsewhere on this tank, spending $350 vs $440 isn't trivial.
Thinking about intakes and outflows: would having both intakes in the back corner (can't be the very back corner given how the top brace of the aquarium curves, i think its too wide for the inflow tubes) at different hights, and then one outflow as a spraybar at the top on one side, and then other as a lily pipe about midway down the tank (and hooked up to the reactor) would give good flow? The two outflows would be directed crossways from one another but at different levels in the tank. The spraybar would provide surface agitation.