Tough call, but in the end, I don't see you regretting any of those options. That is the beauty of being an Audioholic - you are choosing between very good options.
However, I am disappointed that you narrowed it down to 6 options. Surely, no audio decision can be that simple! Let me help you out:
7. one E15HP for now
8. two E15HP's (WAF permitting)
Here's my logic - dimensions are
E15HP - 17"(W) x 19 " (H )x 19"(D) - (20-1/2" D with grille)
F15HP - 19"(W) x 19 " (H )x 20 "(D) - (21-1/2" D with grille)
Only an inch here and two there...that sounds pretty close, but remember, this is a volume. For volume we get:
E15HP - 6,944.5 cu. in.
F15HP - 7,761.5 cu. in.
So the F15HP is actually 11.8% (almost 1/8th) larger (calculated with grill installed), while offering 0.5dB more SPL at 20Hz.
0.5dB isn't much, so the question is whether or not the smaller size would help with WAF for the second sub.
I left pricing out because I see Rythmik is running a special on the black oak F15HP. not knowing which you want, I don't know if you will save money with the E15HP or not.