Closed Deck vs Open Deck was a major issue about 5 years ago before the WRX hit. Mike Shield's of SPD wrote a treatise on why his company would not develop a turbo kit for the EJ25, and a large part of it was the open deck design. This set the stage for a long series of debates about it, and basically gave the open deck a bad name.
But it is apples and oranges to compare the old EJ25 with the EJ20 open decks. One major thing is the bore size. The smaller bore of the EJ20 is far less likely to flex. The other part of it is the bottom end and crank girdles. The EJ20 was designed as a turbo engine that was going to see significantly higher stress than the EJ25.
Now, if you are really going for broke in the power department, a semi-closed or closed deck engine would be a good option. But for 400-450chp, it really is a non-issue. On an Ej20, rods an other things are going to be the weak link before the deck is an issue. And even then I have seen semiclosed deck engines blow cylinder walls! As in a 3 inch long split in the cylinder.
If you are swapping for a swap, and are going to run it near stock or slightly tuned, open deck is fine. IMO, the '97-98 JDM WRX engines are great choices, and pretty easy to find. The heads flow great, and they are really solid engines. They are EJ20K's that are stronger than the EJ20G open deck engines that continued in the world market. If you do a search for Mike's old article (it also may still appear on the SPD site) he goes into some of the differences in spec on the engines as they evolved. Actually, for WRX tech, SPD's site is a great resource...