I personally prefer Portal of Power, but that's because of the story behind it. Not that I don't like the Infinity base- I actually love it a lot, especially since the toys are exactly that in the game: toys, which leaves more imagination to the player... Something games these days sorely lack...
For purely the "base", with no extra backstory or otherwise, the PoP is my choice since it looks much cooler- but the concept behind the Infinity Base is much, much more engrossing in practice to me. Like... The STORY of the PoP is cooler to me, but the way that you USE these bases and the concepts they unlock, I think the Infinity Base is much more awesome in that regard.
I find that the idea of having the toys being toys in the game is much more exciting. It's simple, and nostalgic, and just works. Things don't need to be complex or deep to be fun, which I feel is where Skylanders keeps going wrong. The toys idea in Skylanders doesn't work and feels kind of fake in Giants because you're getting more and more new Skylanders which don't work with the old games(Portals of Power are canonically able to travel time as confirmed by the Quiz Quest Book- so you can't just say "they woke up later", there's a plothole), and you don't really feel like the Skylanders react to you. Not to mention ridiculous Series 1/2/LightCores ruining Suspension of Disbelief. Infinity works with your imagination, and makes better sense since you can seriously play with your toys anywhere, thanks to the backwards compatability.
Sorry for the somewhat pointless, off-topic rant, but I'm bored, and typing one big rant is easier than 50 reply posts
... You edited the first bit with the wrong word, Sherriff >:3