Hello mate,
Well, I have asked myself the same question. And have looked through all of the specs of the bikes currently available, and have concluded that the MONSTER M125 seems like the best choice. The seat height is 4" higher than the Thump and it just looks better built. But it really depends on how tall you are. I am 6'4" so need the extra room. The MONSTER came about after the Thump as I understand it, and is in direct competition with the Thumpstar, but I havn't ridden either, but I have seen many Thumps "in the flesh" and they are OK. But like I say, the MONSTER M125 just looks better built. But if you are spending that much £££, don't buy from a picture, go and have a look at these bikes and sit on them and listen to them, and if you can, ride both of them, then you will know what is best for you.
Good luck
James