Phantasy Star Portable is pretty much just Phantasy Star Universe and its expansion pack Ambition of the Illuminus, but with some different things like levels, characters etc. Basically same world as PSU, but different storyline kind of thing.
It does looks great for the PSP and shame it isn't online, so you'll have to find friends who have this for some decent wireless co-op play. Still fun nevertheless, especially if you enjoyed it's bigger Console brothers.
If you want a portable one that "is" online, then wait for Phantasy Star Zero for the DS. This is the one I am waiting for as it pretty much tries to go back to the roots that was Phantasy Star Online (The best one!).