There are some difference in optical configuration between powermates and barlows - the TV website explains, but here's also a link to a cloudy nights review
http://www.cloudynights.com/item.php?item_id=86
In short, powermates aren't barlows. Other differences to be aware of are if your use an extension tube on your barlow it will increase it's amplification for imaging. If you do the same with a 2.5x powermate it will actually slightly decrease amplification - but each powermate is unique, the 4x will slightly increase magnification with extension tube and the 5x powermate will act much more like a barlow increasing amplification significantly with an extension tube.
I can't compare with a Celestron Ultima barlow, but confess to being somewhat blown away by the optical quality of the 2.5x powermate the first time I used it, comparing it with cheaper barlow like the GSO 2x barlow and Orion 3x barlow.
In the end the biggest driver for powermates will be when you need to get the higher amplification factors such as 4 x and 5x as I'm not aware of barlows being made at this amplification (expect there'd be serious quality issues if they did). If 2x is what you want, no doubt the ultima barlow will serve you well.
cheers,