Can someone who does both give me some info on it? Such as do you attend the classes both places with different coaches of just one? Does it improve your game? 
I did both for a while at two different gyms. Training strictly BJJ both in a gi and without helped my ground game tremendously. I became more technical as a result.
No generally not unless the person is very adept at picking things up. Problem with transient martial arts students is the never stay long enough to get grounded in any one thing. I have guys that come to Judo thinking they will pick up a couple throws for their MMA game or from our striking program. It never works out and they don't stay, they just don't have the discipline. A lot of instructors look down on it also because for new guys it gets confusing.
Outkaster - No generally not unless the person is very adept at picking things up. Problem with transient martial arts students is the never stay long enough to get grounded in any one thing. I have guys that come to Judo thinking they will pick up a couple throws for their MMA game or from our striking program. It never works out and they don't stay, they just don't have the discipline. A lot of instructors look down on it also because for new guys it gets confusing.In OP's case, he is an am my fighter who will probably benefit a great deal. I do agree that martial arts students are fleeting but if they stay consistent, it makes a huge difference to cross train. I trained at an MMA gym for 3 years and cross trained at a boxing gym before I started attending a BJJ school and couldn't recommend it more as long as the school and instructors are reputable.
That might be an exception but generally it doesn't work out so well unless you are grounded in something very well. I can't tell you how many guys in MMA class don't stay and we have a good instructional staff. No one is dedicated to Martial Arts. I thought it was bad in the 80's but now it's even worse.
Thanks to both of you, good insight. I will vote both of you up when I can. The reason I ask is I'd won 3 smoker events by sub before my last fight so I was confident I'd have no problem going to the ground. Once we got there though, my opponent had better bjj and got the sub. He told me after the fight he attended a pure bjj academy and supplemented it with a muay thai school vs an all in one mma gym.
esque - The grappling was way better at the BJJ school. More humility and less fake toughness as well.Agreed
Remember your body type and how you "think" about fighting is going to dictate how well you do standing and grappling, especially if your base was in another system as it was for me. Striking to me is something so instinctual that I don't have to think about distances too much. I know how to kick really well so I can "see" distances. When it comes to Judo, not so much, it was a bitch of a transition because I was controlling myself AND another opponent. In fact it was the toughest thing I have every done and still is. Judo is tough.
Just think of the guys that are out there, they seem to gravitate to one portion of the MMA game more than another. It is so hard to be good at both. No MMA guy training for fights has time to make a sidekick look good for a year, it's just something they don't do. They have to be well rounded as much as possible. Problem is for most of us there is only so many hours in the day. To be good at anyone thing is hard.
Outkaster - Remember your body type and how you "think" about fighting is going to dictate how well you do standing and grappling, especially if your base was in another system as it was for me. Striking to me is something so instinctual that I don't have to think about distances too much. I know how to kick really well so I can "see" distances. When it comes to Judo, not so much, it was a bitch of a transition because I was controlling myself AND another opponent. In fact it was the toughest thing I have every done and still is. Judo is tough.Thanks, great advice.
Just think of the guys that are out there, they seem to gravitate to one portion of the MMA game more than another. It is so hard to be good at both. No MMA guy training for fights has time to make a sidekick look good for a year, it's just something they don't do. They have to be well rounded as much as possible. Problem is for most of us there is only so many hours in the day. To be good at anyone thing is hard.