The larger battery will last way longer, but I wouldn't worry about overclocking.
I was overclocking, and apart from a marginally higher Quadrant score(which has absolutely zero real-world relevance), I didn't care for the instability extra battery drain.
I don't even overclock on my gaming PC any more. When you can get such fast parts for so little money today, it just isn't worth the hassle of having to play with settings, worry about cooling, dealing with instability or shortening the life of the components... all just to be able to say that my game went from 98fps to 104fps... Both of which are far beyond the ability of the human eye or brain to detect.
I tried overclocking on the DX, but it caused random reboots and shortened the battery life a bit, and in the end, the "performance" gain was placebo at best...
These are no longer the days of the Celeron 300a, where you can get an instant 450Mhz+ chip and huge performance gain by changing a multiplier. The law of diminishing returns has kicked in, and in a phone where heat and energy consumption are critical, the payoff just isn't there.
If it were me, I would just toss in the large battery and call it a day...