I don't know too much specifically about Maines or Charolais, but I think you'd like the F1s out of any of those crosses as long as you were smart about the sire selection within the breed. I will say that I have a pretty sting opinion about which breed has the most potential to make the best females, and that's gonna be Simmental.
I'm sure you can find genetic lines within either the Maine or Charolais gene pool that will be more maternal than others, but in general as a breed, the Simmental crosses are gonna make heavier milking, more productive replacements. The 734 bloodlines are gonna work best on smaller framed, easy fleshing cattle with a lot of body. We've had some Machos and have one 1/2 blood daughter in our herd, and she does a real good job. Those cattle can get a little big & hard for my tastes, but that's usually when you get closer to the purebred % or use em on big framed, harder doing types. I don't have a lot of experience with many other 734 sons because their numbers aren't good enough for what we do, but I think some of those are interesting, especially if they're out of a good cow family.
There's an old school Simmie bull that you see in a LOT of pedigrees, and the reason is because the cows are good...Nichols Legacy G151. Our best cows are easily the Legacys, and we probably have more of them than any other bull right now. The purebred guys aren't using him anymore because there's always something "better" coming along, but he's a proven, smart choice. I've also seen some of them that are way too tall and narrow to work, so don't use him on frail, shallow, narrow or light boned cows and expect him to change them. But use him on deep bodied, fleshy cows with plenty of bone and then keep every heifer you get! Legacy is also the sire of Dream On and Shear Force, so start looking at pedigrees, and it doesn't take long to start to realize his influence. Not to mention how many cattle you find out of other sides and Legacy daughters...that's where I like to see him most.
If you truly believe every generation should be better than the one before, look for some Ellingson Legacy M221 semen. They also call that bull "Olie" and ABS carries him, but he's dead now and there's still some demand, and I think the semen went up...I've heard $50. You might find some for less if you know people, and there are some sons out there, too. We're using one called 3C Southbound that was the Lot 1 bull at Christensen's sale in 2010, and he's doing a real good job...we're very pleased.
Upgrade is the most popular "Olie" son, and he'll prob add more power and look than most of the others. We're waiting to see how his daughters perform, and when I say "we" I mean we the breed. We at home just had our first one born this year, but we'll use him more and I'm flushing a 1/2 blood cow to him this spring. Be mindful of structure...they can get a little straight, but the good ones are pretty good.
They're topping a lot of bull sales these days...I think he'll be the next Dream On for the Simmie deal.