The real question should be: Do you like Batsugun much if at all? Or do you like Lord of Thunder more? If you're going to do a trade based on game value, Batsugun is worth more (maybe ask him for a second game that is common but fun or cash to even out the trade value?) but if you're going to trade based on game replay value, then don't sweat too much on if it's a good deal or not.
I haven't played Batsugun as Saturn wasn't my strong point so I don't know if you're getting good gameplay value in trade.