It's all about preference dude! A lot of the time you might just have to rely on a brands reputation to at least know it's not gonna be crap haha. The big brands all mostly have solid playability and it really just comes down to preference between players (some players prefer light damas, some prefer heavy, etc.) I think if he is your favourite player you should take the plunge, show your support and get yourself a new dama!
Please note: I have never played a sam merkel pro model kendama and have no context of its playability haha.
And while sometimes you might need to purchase a dama without actually knowing how it plays, hearing other people's opinions can be really helpful! If you end up getting it, post your own review!
Cheers