Eng - on the 3/4/500 737 BA use (what I believe to be the 'norm') ie 5 and 15. I think I can remember a 25 option on the 200 series?
The whole issue is complicated by terrain around the airfield as well as tailscrape risk and runway length (bet you wish you hadn't asked
)
The shortest take-off run (and therefore Runway limited TOW) will generally be with the highest flap setting (up to the point where little extra lift is derived - 25 I guess on the 737?)
Flap retraction will begin at 'acceleration altitude' (generally 1500' aal in BA 737 world) and starts at or above V2+15 for both flap settings, having climbed at a suitable speed (probably around ?V2+20? ish - well below limiting speed for that setting). The CLIMB gradient at F25 is less than that at F15 due to the extra drag. Therefore terrain may be a problem in the climb-out path. Thus the gain in max T/Off wt at F25 MIGHT not be significant, as MTOW will have to be reduced to allow terrain clearance as above. IE You might be able to get airborne with 2 tonnes extra at XXX in the valley with F25 but you would smack into the big hills 4 miles off the end of the runway as you flew a shallow, accelerating clean-up!
Told you it ain't easy!