I've not experienced any roughness or vibration with SOHC VG's in the past. Bang for buck they are almost unbeatable given their reliability but they are getting a bit old now here so parts are becoming harder to get. There i doubt it's a problem. I have one in my shed that's only got 70,000 miles on it but I doubt I'll ever use it in a swap now because of that. Long term, in the UK, it's probably not a wise choice anymore.

Remember, a lot of 510 owners there run them so there is a wealth of knowledge about them over there. They were probabably the number one swap into a 510 before cheap SR's became available.

As for VG vs VQ...in terms of technology you can't compare as there's 20 years between them but in terms of size the VQ is undoubtedly wider. It cannot fail to be as it's DOHC. Both will fit in the same engine bay but you'll actually be able to fit you hand down the side of a SOHC Compare VG30ET powered 510 with Dave Lum's VG30DETT powered one and you can see what I mean!

BTW, there were no AWD S13's. All of Nissan's small AWD platforms suitable for cars are based on a transverse engine layout apart from the Skyline. The small truck setups such as D21 are a bit more suited to off road than on road and aren't really very performance orientated.

-Ed