I have recently thought a lot about this myself, and wasn't going to post about it here, but you might find this interesting.
Oddly enough, I just purchased a new bike, went from a 53/39/30 with a 11-34 cassette (tri-cross triple, set up stock) to an '01 poprad frame built up with a 46/36 and a 12-25. I live in a pretty hilly area and am not an experienced rider at all - before a month ago I'd been on a bicycle less than a dozen times in the previous 15 years of my life, and I'm 30. While I do want to race cross once I get my fitness level up, I also want to be able to cruise around, and really would prefer not to have two bikes. Yet. It's pretty much inevitable I'm going to have a second bike at some point in time, but having two cross bikes when I like one better just seems extremely redundant and a waste of (sadly) limited resources. Anyway, I've been looking at a few options, and last night made myself a spreadsheet.
I'm a total new guy to the biking world, but I figure if the cadence is equal, the speed will tell me which gear is easier/harder to pedal in. There's probably a different metric I could be using - and I'd be happy to take any suggestions. If I call something by the wrong name, be patient.
The first sheet, I wanted to look at which would be more dramatic of a change - changing to a smaller chainring on the smaller crank, or getting a larger cassette. It looks like moving from a 36 to a 34 tooth chainring in front has just about the exact same effect on speed as moving from a 25 tooth to a 27 tooth cassette. 36/27 at a cadence of 60 = 8.9179mph, and 34/25 at a cadence of 60 = 9.0963. That's pretty damn close, and I'd much rather change out the cassette than start f'n with the crankset - but it gets even more effective when you look further.
From Sheldon Brown regarding capacity of a derailleur:
Manufacturers specify this fairly conservatively. They must do so, because they have to assume that some of their derailers will be sold to incompetent cyclists, who will abuse their drive trains by using the smallest chainwheel with the smaller rear sprockets.
Competent riders can considerably exceed the official rated capacity, since they will not misuse the granny ring by running it with the smaller rear sprockets, so it doesn't matter if the chain hangs slack in those gears.
Rear derailers are also commonly designed for a particular maximum size rear sprocket. If you exceed this size, by too much, the jockey pulley may rub against the sprocket when using the lowest gear.
Rated maximum rear sprocket size, however, is also commonly much lower than what actually works. For instance, Shimano's models designated as "road" derailers are generally listed for a "maximum" sprocket of 27 teeth...because 27 teeth is the largest size that they make in a designated "road" cassette. However, in almost all cases, these derailers, even the short-cage models, will handle rear sprockets as large as 30 teeth in practice. (This somewhat depends on the design of the frame's derailer hanger, so once in a while you will find a particular installation where you can't use a 30, but I've never seen one where a 28 wouldn't work.
So now I'm pretty much positive I'm going to change the cassette (because really, becoming a stronger rider just sounds like a lot of work), it's just a matter of which cassette. I compared three cassettes throughout the full range of gears at cadences of 45, 60, and 90. Not that I'm pushing 90 yet, but hey, a guy can dream, right? The three are as follows (from a quick search last night, any suggestions are welcome.) the comparisons are on sheet 2 of the spreadsheet I linked earlier.
Ultegra 12-27 - 210g, $90 at my LBS, $64 shipped online. I'd really rather buy from the LBS, but dammit, that's $26.
Harris Cycling (off SB's site, shimano cassettes rearranged to gear sizes not available from shimano) 13-30 - 310g, $113
SRAM PG-950 - 370g, $31 + shipping.
Random thought - there's only two gears on the small chainring that are slower than the slowest gear on the large chainring. I think I should spend most of my time on the larger chainring, especially if I get the 13-30 after reading what Sheldon had to say.
If anybody's actually made it this far through my rambling rookie gearing manifesto, I'd love some feedback on my decision. First off, I'd like to congratulate you, as my girlfriend races on a cross team and sells bikes for a living, and I think she's growing tired of me sharing stuff I've figured out that anyone who knows anything about bikes sees as common knowledge. She's gracious though.
I'm not a huge weight weenie, but if I'm going to be a weight weenie about anything, it really seems to make sense to worry about the weight of something that you're spinning around with your legs as well as moving forward - and the difference between the sram and the ultegra is a third of a pound. That seems like a lot. A decent burger, even. It doesn't seem remotely worth it for the extra 20 or so bucks it would cost for the ultegra if I buy it online and then slink into the LBS to have it installed - or maybe I'll google it and give it a go myself. I'll work on the drivetrain, worst that happens is that I can't go. I had the LBS install interrupter brakes. Not being able to stop would suck
So, that leaves me with the 12-27 or the 13-30. I'm not concerned with the derailleur capacity, as I'd spend 85% of my time on the 46 tooth ring - and 46/30 is a slower gear than everything on my current setup except the 36/25, my lowest. I'd only be using the 36 inch chainring for the lowest three gears, which don't leave too much slack at all. I'm not concerned with the difference between 12 and 13 - and if I was, harris has a 12-30 as well. At a cadence of 90, 46/13 is over 35 mph. I don't go faster than that.
There are two distinct differences I'd love to hear your thoughts on. Weight and price. $113 isn't prohibitive, but it's over double the cost of the ultegra. In addition, it's also 100 grams heavier. My main question is how much the weight will matter in a cassette - because I like the gearing of the 13-30 a lot better. But hell, after I pay the LBS $25 to install it, I'll have shelled out $138 to change the cassette on a bike I paid $500 for. This seems silly to me, and that money could go toward a carbon fork so I don't feel like I'm riding an electric hand mixer when I brake hard.
Thoughts are welcome if you've made it this far, and if I've made any incorrect assumptions you're more than welcome to correct me. I have a lot to learn. And wow, I just puked up a whole lot about this.