Why not both? Er, that is, use magnets to give one beastie double duty. That way, if you're unimpressed with one, you can always run it as the other.
I think there are some good suggestions on how to properly magnetize this kit in the forums. (At least, I remember seeing something to that effect...)
I actually had a similar dillema and couldn't decide so I just bought 2 kits and made one of each. Admittedly these days My Thundertusk see's more action that the Stonehorn. Stonehorn is for when I'm feeling silly and want a mess around game with friends. If I want to increase my chances of beasting my opponent 6 ways from sunday the Thundertusk is usually the better option for me and my army. The Thundertusk tends to be universally useful whereas the Stonehorn seems to be a bit more limited in my experience. 99% of the time my opponents WM's see my Stonehorn and go "aaah! Hello!" followed by a large BOOOM!
Really? Isn't the Thundertusk so much easier to take down with warmachines? I actually play a Stonehorn just to swallow cannonballs and it works pretty well. His halving multiple wounds rule is pretty neat. He can take quite the punishment from warmachines and your opponent really has to dedicate himself to bringing him down this way. All the while, my other units remain unscathed (the main reason I take him). Hell, sometimes he even reaches the enemy and then the fun really starts!
D...
Yeah the Thundertusk is usually easier to kill however my opponent being a complete plank doesn't see it as much of a threat. He usually focuses on the horde of Ogre's it's supporting. He wonders why he has lost to my ogres 4 times in a row lol. My opponents usually see the Stonehorn as more of a threat because of the half multiple wounds rule. They take him out so he can't kill anything important. Generally my Thundertusk doesn't see a massive amount of combat unless I'm sure he wont be taken out before I can strike.
It obviously depends on the army i'm playing against but I'd say for every 3 battles the TT see's the ST only see's one. It's my personal preference though. I'm quite a defensive player and a Frenzied T6 Monster doesn't really feel defensive to me. I focus on turning the bulk of my army into a dirty anvil / hammer so the TT plays into the stratergy. I also don't like the Earth Shattering Charge rule all that much, potentially less attacks on the charge than it's statline.... To be fair I doubt we will ever see the end of this debate
