If you are looking to acquire power to play with and cost is a factor, I'd suggest HP or MP cards. The value isn't in the condition and playing with them won't hurt it at all.
I have the monies to buy them in NM condition, but I would prefer to get a SP card since the cost difference is quite noticeable. I just don't know if going into the MP/HP category is worth it to save some money. Just asking for insight if possible since I don't have any older cards that are beyond SP (:
If cost difference is relevant to you, MP/HP is probably worth. It's possible that you put your SP stuff on the border of MP by playing with them for a little while, and you'll lose the premium you paid (despite them appreciating above what you paid anyway).