For what you want to do the Mackie will do a very good job. I had one of the original 1604's and used it for many years just mixing 8 stereo line level synths. The newer models should only be better still. I have used the Mic Pres too for many a multitrack session and got perfect results with nice microphones. And that was with the first 1604, the new Mic Pres are quite better now since then too.
It should match your incoming levels well. Add or subtract any gain as required. The Mic Pre should have a fairly neutral sound and be clean if operated well within its limits. It should pass input signals out and onto your audio interface very transparently, cleanly and quietly too. The EQ comes in handy to make some slight broad changes on the way through. The Low Cut filters are sometimes just the ticket for taking excessive bottom end out of some synth patches bringing them back to normality. Make sure you route some of your synths at least into the Mic input channels so you get the use of the HPF if you want it.
(The fatter more analog sounding synths are good on these inputs. Synths with a wide range of output levels are also good on the Mic input channels too.) I downloaded the manual and had a look at the block diagram. On channels 1-8 the Line inputs on the Mic/Line channels are still not a true line input. They are still attenuating and running into the Mic Pre. The Insert Returns are stlll a good place to feed a very high line level signal into the mixer and totally avoid the Mic pre on those inputs if you need that like for a very high quality synth that just needs to go straight through to the output with minimal interference. Even if you use the Insert Returns the HPF still operates just after that signal point. Inputs 9-16 line INS are true line inputs with no Mic pre involved.
(9/10 and 11/12 do have a mono Mic Pre that can be used for extra gain on a mono signal input which is interesting the way they have organised those inputs) Depsite where they are being built they must be still made to a spec and if the spec is good then it wil satisfy your requirements well and effeciently. It would add the least colour of the two and just route your audio nicely. No FX suits you too. I personally find adding some reverb from the Samson on the way through to the DAW can be beneficial sometimes. Different sound, different colour etc.