Hey there,
If you have a choice, I would go with the Seminole for the following reasons:
1. Cheaper. PA-44s are always less/hr than a PA-34. You ticket looks the same if you did it in a Seminole or a Seneca so save your $$$. If the Seminole is $20 cheaper, over a 8 hour course thats $160, enough to buy another hour (every hour counts!).
2. Less complicated. Starting with the II, the Seneca's were turbocharged, and that's just one more thing to hassle with on a checkride. The II's especially had fixed wastegates on the turbochargers, and it was more of a challenge to fine tune the MPs, especially when dealing with SE ops. Not that you couldn't handle it, but why not stack the deck in your favor? With Seminole you push the power to the stop and forget about it.
3. If you want a HP endorsement, take the money you saved by flying the Seminole and go fly a 182 for .5.
JMHO,
Richman