I just bought the s150. I didn't own a cycling computer, so I wanted those functions in addition to the HR. I got it from Performance. Their price is $74.95 and I found a 10% off coupon, plus about $8.50 shipping. I just got it yesterday and used it today for the first time.
It works really well, isn't hard to set up (but I did read the manual carefully) and, given it's only day one, I am very pleased. My understanding is that the 120 and 150 are identical, except for the cycling functions. The manual is even the same for both, with grey shaded boxes that cover the 150's biking functions. There are some limits to the bike functions: 1) stores only the most recent ride info (so you have to transfer it, manually, to a computer or other log to save it); 2) no cadence feature; 3) no calories burned feature.
The HR is very comfortable and other posts indicate it is very accurate. I have to say it was interesting to know my exact speed, HR, distance, etc...no more fooling myself
I would say if your current cycling computer meets your needs, then the s120 maybe a the best choice, depending on price. If the price is about the same, then get the s150 and you can put it's cycling functions to use with your other bikes.
Hope that helps.