Same difference...... you are looking for the main outs post effects. I would actually prefer to have exactly what the LR is outputting to the mains. Being post fader, the signal output is almost always more uniform and balanced and makes for better 2 track recording.
Whether you use matrix, subs, aux, or whatever the board or engineer calls it......you want to be post all effects and all faders for a 2 track recording. Some would argue that point about effects, compression, eq, and such, and how it might adversly affect the recording, but I've found that unless the compression is ridiculous, it's going to sound much better than a dry pre mixed signal any day.
Not the same at all. There is a big difference between an aux out, a subgroup out, a matrix out, a B mix out, and the mains out. Yes, the simple answer for recording is to take the mains out, but most of the time the main outs are already spoken for...for what they are intended...the mains. An aux out, even a stereo mix, isn't going to yield you the best results since, even if the engineer sets every channel to unity on the aux send of each channel, you still aren't going to get the effects returns on your tape, unless, of course, the engineer brings all his effects back in channels and remembers to send the effects out on the tape aux feed. Subgroup outputs aren't going to do you much good for a recording, since, generally speaking, they are used for grouping instruments, vocals, etc, and, again, you aren't going to get effects. Plus, most small format consoles don't even have subgroup outputs (none on the Midas Venice, the small format console flavor of the month). The B mix output is a safe bet to get a decent tape, but the B mix usually follows the main L and R faders...which is OK, but any changes made to the mains (L & R) will effect your recording. My solution is this......I set up a stereo matrix mix of all my subgroup or VCA outputs, plus all my effects returns, and record from that. That way, if I add 3 or 4db at the end of a song to the live mix, it doesn't effect the recording. Of course, this can only be done with a board with a true matrix section (aka a PM3/4/5K, XL, etc...), and with an engineer who is willing to work with you to figure out what you want on the tape.
I'm not exactly sure what you are refering to when you mention "effects, compression, eq, and such, and how it might adversly affect the recording", but any channel eq change is going to effect a board recording on any output from the board. The main EQ isn't going to affect your tape at all since you are getting a signal before the signal even hits the EQ.