The Beta and Monty have the highest quality frames out there in the current trials market, being monocoque aluminum jobs made by Verlicchi in Italy. This makes them good mounts for the heavier rider who does not have asperations or winning the champ line.
The beta motor gives you one more gear on top (6 in it vs 5 in the monty), but has the left side kicker. Not a big deal, just pointing out the difference. Both make great linear power and are rather neutral handling bikes. The Beta, being Italian, is a bit sexier bike than the rather staid Monty. Look a the beta from the riding position - the tank and frame area are shaped like a woman's body.
I "feel" better about riding the Honda made product, but would take a Beta as well no problem. Maybe one day if I move up to the Advanced line I may look at a Sherco or GG Pro, but not until then. I'm 5'11" and 165 BTW, just (legitamately) moving up to the Int line.
Good luck with your choice! Even if you do ride all the bikes, it is hard being a beginner to really feel the differences and what they mean to you if you are a new rider. If going used, focus on Condition, not price. Also on the Monty I'd say get an '00 or later and the Beta an '01 or later. There is a hardly ridden '01 Beta sitting over here at the local dealer in Colorado Springs (Apex) BTW for $3300.