Why a 12/25 cassette when...

 That doesn't make it for me. I need a setup for ALL hills. :-)

I got up there with an 11/23... So maybe "most hills" was an understatement.

Hey nice pix.

THAT setup you have looks like a very nice combination of gearing!

"oh honey I need to buy something else..." Nah that's not going to fly right now

53/39 with an 11-23 seems to work fine for me for that .......

The most common gearings are 11-23 and 12-25.

For your situation, sounds like your current pairing doesn't work too well because of terrain, and effort. Great. So go 11-25. Simple, easy solution. Lots of gears, and a pretty good top end.

Btw, the only advantage a 13-25 will provide you is closer gear ratios, sorta, and an EASIER top end?!  I don't get it. IMHO, you always want the 11 on a downhill, and you (most) always want the 25 for an uphill.

On the downside, if you ride a 13 on the downhill, you could gear out, AND lose distance and time. I know of no triathletes that ride a 13, unless they have a triple.

 I guess that's me. :-) However, over the years I have come to learn to be a very goo descender. I use a low profile (road bike) tuck position developed over years decending in the mountains. I go into it as soon as I spin out.  NO NONE has ever passed me going down hill. And I fly past many people. The one criticism that is valid is that holding a tight tuck uses a lot of energy. But I do get a kick watching people having to pass me a second or third time going uphill only to have me pass them going down hill. I love going fast downhill. 

And a 13 will not really let you do that ........ I regularly hit 35 - 40 on most of my rides on downhills and as much as 42 a lot.  Which in my 53/11 is going pretty hard at around 100 rpm.  Can't do that with a 13 ...... especially if you're running a 50 compact.

Thanks for the feedback all. 

But the big thing is this, I live on the FL east coast, my regular "hills" are 1/4 mile long ups and downs on the couple 4-5% grade Causeway bridges we have here with or without a decent tail-headwind.  I have been using my existing OEM 12-23 for everything and have only used the 53/12 once in a race (20-30 mph tailwind & down a bridge) for a total of 1.5 miles, and a handful of times training only for short distances, so for my situation (IMHO) it is a wasted gear.  Now I've used the 39/23 way more and in a Clermont (central FL) race in 06 I could have used a 25 to 27 gear but not the top end.  As I stated in the OP, ~33 mph top end (53-13) was fine for me, I'll rest and stay low the rest of the way!

If I lived in an area that had sustainable downhill runs or was looking at a course like LP, yes a 12-25 or compact/11-23 would probably the ticket.  I guess I'm just being over analytical about it like I usually am!

 You are right, there is no way I can hit those on gentle downhills. But so long as the hill (or mountain) has broad sweeping turns (or is straight) I easily blow past 40 mph. I will admit that even on perfectly trued wheels, I sit up at 55. I just don't have the nerves for anything faster.