I don't notice any midrange benefits to the larger speaker...if anything, I think the F1's image just a bit better and produce a slightly more pleasing soundstage...which sub are considering though? The 10" MKIII Titanic? That's got over 200 watts if memory serves, and is worth every penny.
I would go with that in heart beat over the F2's. But keep in mind, you'll have to acquire stands, and do a bit of work to integrate the sub into your system...
A word of advice...don't rush, there's nothing wrong with saving up money and waiting just a bit longer for something even more...in the long run it's cheaper than buying and upgrading every year or two.