First off, I want to lay out some facts. It is a fact that in stock form, the SOHC heads flow a little bit better through the mid-rpm range (4500-5500) while the DOHC heads flow slightly better up top (5500-6500). It's really pretty close and pretty much a wash.
Another fact, as was mentioned, is that if you overport either of these heads, they will suck. It's about port velocity and not about reaching maximum flow. With the right turbo and a mild port, you can pretty easily hit 350whp on either head. This is on stock cams. If you want to read a good discussion on cams, search for Graham and Xephyr in this forum using cams as the search word. I think BOY was in on the "conversation" as well, as was likely Drac9, No-Coast Punk, and myself.
I think the reason you see more people go with the DOHC Ej25 heads is twofold. The first is that they sell for about $100 cheaper than SOHC Ej25 heads. I think the other reason is that because they are cast on an old WRX casting, they already have the bosses for oil and coolant and are just a drill and a tap away from a very stock set up.
Using the SOHC heads requires custom oil and coolant (which isn't rocket science as many members have seen). The other thing is that a stock WRX uppipe doesn't clear them properly, so you are left to manipulate things quite a bit or do something other than use a turbo crossmember and factory style uppipe and downpipe pieces. But again, for a creative and resourceful person, it's not a bit deal.
On the whole Ej22T thing, that's really a personal choice. 350whp is not record breaking. It's not anything that is going to stress the Ej22T. I wouldn't go around saying that 500whp is well within the design limitations of the Ej22t, but 350 is. There's just no point to argue Ej20 versus Ej22 at those power levels. Hell, even an Ej22E open deck NA engine (with proper pistons) will be fine at 350whp and those "matchstick" rods won't matter. It's closer to 400whp or in situations of crappy tunes where that matters.
If it was my car, I would build it on what you got. It's about the same cost to do it either way. Most of the names I have dropped in this thread have done 350whp AFI builds. Do some searching on the history here and you will see it's not that hard. I just stress one thing: tune and fuel, tune and fuel, tune and fuel. I guess that's 2 things. oh well...