Why not both? Hire forrestv full-time to work on P2Pool and spin up another group (maybe headed by Luke-jr?) to improve GBT. Bitcointalk is making about $5k in profits per week (based on some back-of-my-hand estimates on undisclosed expenses such as the sysadmin that was hired along with the known expenses such as mods), which gives plenty of room to work with without even touching what has been stored up. Bitcointalk could currently sustain two full-time devs at $120k/year.
I'm not sure that this issue is important enough to warrant two full-time devs.
Also, I don't want to eat into the reserves too much, especially with the forum software project unfinished. (The forum currently operates at a monthly deficit of ~90 BTC due to the software project.) I think that one full-time dev and some occasional bounties could be safely supported now, but not more.
As for which projects to support... I'd like to see more discussion (especially from Bitcoin experts and core devs) on where additional resources would be most useful.