Why would one ever buy a 12-25 cassette when an 11-25 is available?
If you want to maximize your max. speed, go for an 11 tooth cog. If you want to maximize your average speed, unless you're a pro you probably are better off without it. Even cruising at 40km/h does not require and 11 tooth cog.

For example, take a look at this table, showing cruising1 speeds for a 11- 21 tooth cassette:


And compare to this table for a 12 - 25 tooth cassette:



You can see that the 11 tooth cog is only really useful for cruising speeds that riders at the professional level can maintain, or for short bursts. So, like I said, if you aren't trying to break your own max speed record, you may be better off with a 12 tooth cog.

(Tables screen captured from bikecalc.com. Check it out.)

I remember reading a while back about how most riders can't maintain a high enough wattage output to make an 11 tooth cog worthwhile, but I couldn't turn it up. Maybe another contributor will post it.

1 Cruising here means pedalling at 90 r.p.m.