Hi there! I'm also new to the sport and I also happen to have an MR100. I'd say I started sometime around summer of this year?
Anyway, I kept hearing from everyone that the first upgrade you should do is always exchange stock barrel. I did just that and purchased myself a J & J ceramic barrel 14".
Sadly, I haven't been able to get it to perform at optimal efficiency. I either have the balls shooting to low, to they'll just kind of flop out the barrel or they are coming out too strong that they wiz to the sides. I've got to go to a chronometer and find the right settings.
As for which is better...a 14" or a 16". I couldn't really tell you. I've heard so many things. That a 16" is waaay better than a 12" but that a 14" and a 16" are practically identical. I went ahead and got the 14" since I didn't want to be carrying a cane with me. My friend uses a 16" with an A5 and it's pretty awesome. He got me from some distance although the ball didn't pop.
Anway, it's really up to you. I'd go with the 14" (since that's what I got) but maybe you can borrow someones 16" and compare.