..Due to my interest in the Martian Manhunter character I was wondering about picking up JL United...on the other hand the chance to read Shazam in the main JL book is a pull...Which do you prefer?/recommend?

Comparing the first few issues of both run I'll say JLU

Both have thier ups and downs. JLU is better han the main book by comparing thier starts. but the main book has been really good as of late. Go for JLU.

But, if you can spare the dough, why not both?

Justice League United. The story is great is great as well as the roster. The art is good. Nothing to be great though same is the Justice League's art right now in my opinion.

I highly recommend Justice League United. Its fun and the roster is great. The Justice League book is getting a bit better from its early days.

Justice league united has a better story and the line is good, some character development will surely happen, and the space mission which they do is kinda cool. A-man and G-arrow really in the frenemy spot now

Justice League United is the better team book IMO. The roster is ecclectic, the writing is fun and energetic and the stories take a lot of influence from Silver Age and Brozen Age characters. Lemire is using DC's often forgot about history as he well should do. As should DC tbh. Anyway I'd recommend JLU over JL. Though JL is better than when it started, JLU is doing much more right as a teambook. Better team dynamics, better tone, better roster and better character development.

I prefer main.

Beata

Also definitely United. JL is probably Johns' worst work ever.