>>will it make much of a difference if I have some of my or all of my synths connected in stereo channels while mixing?
Possibly not. It depends on how many stereo sources you want to record. On the other hand I guess there's no reason why you couldn't use a stereo channel by putting one mono audio source into the L and another different audio source into the R, to make the stereo channels act as hard-panned mono channels, and using the 'balance' knob as a crossfader between the two if needs be.
The only problem in these cases is that you wont be able to pan these psuedo mono channels - they'll each be locked to hard-left or hard-right. So you couldn't really place something like the bass line into either the L or R of the stereo channels alone as it'd be completely lop-sided (bass should ideally be centrally panned), unless you sacrifice a whole stereo channel for the bass (ie. the bass going down both the L and R channels), or using on of the dedicated mono channels centrally-panned.
Having said that, most synths these days use stereo, so you could use the stereo channels on the mixer for inputting things like stereo pads/strings, other stereo instruments, effects and the like.
Personally I always think people should get as many channels as they can afford when looking at a mixer. Not only enough channels for all of their synths and other instruments, but having additional channels left over for additional creative uses. Things like parallel compression (making a duplicate of a channel and compressing one of the channels, and mixing it back with the original, for a fuller sound), psuedo stereo enhancements (taking a copy of the L and R of a couple of channels, inverting the phase, swapping the channels [ie. L > R, R > L], and mixing them back with the originals for a massive stereo field, albeit not very mono compatible if done excessively), or tape-echo based effects (using a delay on an auxillary effects send, then taking the outputs of the delay and putting running them back down their own channels on the mixer, enabling the use of EQ to sculpt the wet delay, and then gentley feeding this back to the delay auxillary for more feedback), or using the outputs of a reverb device and running the outputs back down an empty couple of channels to sculpt the EQ of the reverb's wetness, or sending it to more effects, etc.
There's no limits of creative things you can do with more channels. This also depends on how many aux sends, stereo returns, tape inputs, direct outs, and busses you have, etc.
See here for an excellent article on this:
http://www.soundonsound.com/sos/1996...xerbusses.html
If you have two inputs
per single mono channel (line/mic input, and a second Mix B input) that's another boon (like an "inline" mixer), as you can pretty much double up the number of channels that can enter the mixer, although only the main channels will normally be able to access the EQ and/or aux sends, etc.
I think anyone that wants to buy a mixer should first research what mixers are capable of, how they can be used, and understanding the terminology so they can understand the feature-set properly before buying one (ie. clearly understanding the differences between pre- and post-fade aux sends, etc.). Then sitting down, count up all their outputs from their synths that they think they might need, and also think what they need out of the mixer itself, ie. whether they would be happy to record things in a multi-pass way, building up the song in the computer bit by bit so they can maximise the features of the mixer for each musical part.. or whether they want to mix
all the music
all together in one, single-pass.
There are some good articles regarding the basic concepts of using a mixer on the SOS websites, in their article archive. Some articles I found using the briefest of searches:-
http://www.soundonsound.com/sos/1997...eranatomy.html
http://www.soundonsound.com/sos/1994...eranatomy.html
I'm sure there are lots more on the site.