I think Alzner easily has as good (and imo better) potential than Franson. Alzner looks like a possible top pairing player with a well rounded game that does a little bit of everything. Yes, he is a very safe pick, but he also has quite high potential imo. I am really not sure why people don't seem to think Alzner can be a top pairing player. What is he missing? Physical dominance? Sure, he isn't the kind of guy who will make forwards keep their head up all the time, but he doesn't need that to be effective defensively. He uses great body position and stickwork to shut down offensive options, and is much more effective defensively than most big hitters. He can certainly handle himself in the corners and in front of the net, and that's plenty enough for me.
Franson can control a game at the WHL level, but this ability to dominate and dictate the flow of a game will not be there in pro hockey. For him to be a top pairing player in the NHL, he will need to be able to do more than he can currently. The WHL game seems like it's played in slow motion to him right now, so it's easy for him to make those great first passes and quarterback the PP and really own the ice when he is out there. When he makes the jump to the AHL, though, where everything moves so much quicker, I think those aspects of his game will be reduced a little, and he is the kind of player who struggles a lot when his confidence is not at its peak. For that reason, I think he will have an adjustment period in the AHL of at least a year, and then again in the NHL. Also, he still has to improve his footspeed and agility and backwards skating to handle better forwards. He is not a bad skater, but he needs to be a better skater in order to ever be a top pairing player.
All in all, I think Alzner is safer, and a much better bet to become a top pairing player than Franson. I expect Franson to be a top four player several years down the road, whereas I expect Alzner to be better than that, sooner.