I personally prefer the S14, they have better handling. S13 parts needs to replace more often then S14 due to the fact that its a bit older. Like Tak said, S13 is for beater, while S14 is for daily driver. They both are easy to work on. S13 are only lighter by like 90 pounds. S14 are shorter and have less cabin space than S13, but are wider. So if youre short on money, just go with S13. They both are pretty good cars though. Theres no big dramatic difference between the 2. Suspension is almost the same.
If you need power, you dont need a SR swap. Just add simple bolt-ons and it would be fine. Theres plenty of basic information on TR and http://www.240sxforums.com/