jdm is lame
sr's are lame
ka's are worse
You know I hate you, lets not go over this one again ;P *coughrbsareawasteofmoneycough*
The Silvia's and 180sx's came stock with the SR20DE, SR20DET and CA18DET (earlier models of the S13). The Pulsar Gti-R had an AWD version of the SR20.
The turbo versions of the SR (and maybe the NA... but I never looked too much into it) came in different versions as the years progressed. The first is known as the "Redtop" SR20DET and came out in the 1991-1993 models. Later on the "Blacktop" SR20 came out for the 1994-1998 180sx. There is very little difference in the two engines they both have the T25 and are visually identical besides the valve covers. I have owned both a redtop and blacktop and I have never felt much of a difference to care about, some people say that internally there are slight differences that would make the blacktop more reliable and I wouldn't rule that out... but in the long run one is obviously going to be cheaper than the other and just depends where you are going with the car.
S14 Silvia SR20DETs are commonly known for the upgraded T28 turbo. Most of the information like the S13 I can probably just regurgitate from a search on Yahoo but since I have only driven one with it very few times and have not have much experience with it means I don't have any opinion about it. It is just common for people to use a ball bearing T28 from an S14 and swap it with the S13s stock T25. Which trims came with the ball bearing and non-bb I wouldn't know either, my guess would be that the Kouki S14's came with the BB T28... but that is just a logical assumption.
The S15 SR20DETs also came with the T28 but featured a six speed tranny. Again, I have never had one installed in any of my vehicles so I can't do much more than spit out what I can search for on the internet. I have driven an S13 with an S15 SR and it was a little odd but worth it I guess if you have the money and it's what you are looking for.
Truthfully with the "Jaaay deee eeeem"-ness, there really isn't much special. Engines you have some really well tested after market support for it, but it costs a lot of money. I am used to SRs and they fit into what I am looking for so I will probably stick with them for a while. However, in my original situation I would have rather just stuck with a stock KA and completely built the suspension first... but my original SOHC KA was on its last leg... so I had to do something. BAMBAM is right though, you can get a DOHC KA for roughly around $200-300 easily. There are probably more people today with SRs and RBs in their S chassis than there are KAs.
JDM doesn't mean too much more than it wasn't sold in the US. I have had lots of crazy ass problems all the time with both SRs... because they are JDM doesn't mean problems magically disappear. Don't even bother saying JDM the only people who really care are usually fanboys and noobs (no offense to any fanboys and noobs here). I have been daily driving SRs for a little over two years now... really it lost its affect a week after it was swapped and i burned a hole into an exhaust gasket (first one out of probably around three).
Really if you want to know the best approach to modifying and having fun with your car, then forget the engine and focus on suspension. It is the easiest and most universal thing pretty much. If someone has an SR then thats cool, but with stock suspension you are still technically just a stock car... maybe a jaaaay deee emmmmm stock car... but stock is still exactly that. And when it comes down to it there are people who take stock cars to the track and will show up plenty of noobs with SRs, RBs, LSx, and whatever else there is out there. Learning to drive the car for what you want to do with it is always the first step. The next step would be begining to modify the car and my suggestion would be suspension.