Both LFS sound editor system and the CSR remixer are both very limited in different ways. If you were to record your engine sound through the entire range of revs then your best bet would be to use CSR, since that uses samples of sounds that you link together. This means that you would have to chop your rev sounds up into different parts, like: idle -> low -> med -> high
Then do the same for the off-throttle sounds. There are a few problems with CSR though. There are no external sounds, so even viewing replays in an external view, you will only hear the sounds as you would from the cockpit.
Using LFS' sound editor system, you get cockpit and external sounds. The problem with LFS is that the recorded samples that you can use are very very limited in length. The length of the exhaust pulse can only be 0.07 seconds maximum. This is because LFS simulates the sound from each cylinder, so it is just a quick pop. You can record tiny little samples that do make a difference, but it's not always a huge difference since there is not a lot that you can change with only 0.07 seconds to play with. Using LFS you can get some very nice sounds, but it doesn't always sound REAL. For the other sounds, such as the starter, blowoff valve and gearshift, you can use longer recordings, but really not by a lot. I think the gearshift can be 0.1 seconds max and the ignition only 0.7 seconds max.
Neither are perfect, it's just what you prefer I guess. Hopefully the devs will do some work on the way sounds work with LFS, as I think that sounds immerse people more than they realise.