In general, Ice Punch > Drain Punch.
The reason for this is simple. M-Lopunny is very frail, and functions as a Glass Cannon more than anything else. It is a high risk high reward Pokemon that can change the flow of the game through hyper offensive maneuvers, that allow for little defensive options like Drain Punch. It just doesn't have the defenses to try and regain HP.
Further, one of the most common switch-ins is Landorus T, which can completely wall your set (save for Return/Frustration) and put a dent in your momentum. So Ice Punch is almost mandatory nowadays, seeing how just about every other team carries Landorus-T.
A common set I've seen used is
Lopuuny (Mega) @ Lopunnite
Adamant Nature
252 Attack / 252 Speed / 4 Special Defense
- High Jump Kick
- Ice Punch
- Fake Out / Power Up Punch
- Return
Having both Fake Out and Power Up Punch leaves you defenseless against common switch ins.
Ice Punch also deals massive damage on Scarf Latios switches which look to Psyshock and KO your M-Lopunny.
An effective tactic to use when you know the opponent has Landorus T is to simply click Power Up Punch, which nullifies Intimidate and then threatens to OHKO offensive variants with Ice Punch. If the opponent switches to Latios, switch into your Ferrothorn / Celesteela (assuming no HP Fire).
