Alot of people have been arguing about which is better, the slime hill or the bowhole. Here is my take on the conflict.
Slime Hill: The slime hill is good if all your after is metal slimes and LMS. This can be frustating, though, because the amount of metals spawned in changes, so some days you'll have better luck than others. The enemies on the slime hill are easy, and don't give enough XP to be worth fighting.
Bowhole: The bowhole is good if you want to fight monsters along with the occasional LMS. The monsters there give a good amount of XP, but the bowhole is bad if you are just hunting LMS. The only plus is that you do not have to beat the game to go there, and is helpful if you are trying to level to fight Corvus.
Grottos are also good places to fight in, since you have the chance of getting new monsters and treasure. I use all of these places alot, starting with the bowhole then going to the slime hill and eventually fighting grotto bosses.