Based on your choices I'm guessing you're on Verizon?
I would actually say neither. Look for an Android phone, here's why:
The Palm Pre Plus, while better and faster than the original Pre, has a more limited user base, and WebOS has a small market (it only exists on the two new palm phones - Pre and Pixi). This means less app development, so less apps available (especially the free ones). It also has less potenial market staying power (the WebOS change was a last stitch effort to save Palm... which sort of worked, though they were just bought by HP who have never done too wonderfully in the cellphone business).
The Omnia II runs Windows Mobile 6.5, the final version of the Windows Mobile operating system. Though the OII has been called the best implementation of Windows Mobile to date, it will also be one of the last. Microsoft will be releasing Windows Phone soon, which is basically a whole new animal (Windows Mobile is widely recognized as mediocre software, which is part of why Microsoft killed it off and created a whole new OS from scratch).
Look for something Android based (it doesn't have to have 'Droid' in the name - just check the specs, it should say what OS it runs there). It's relatively new software, but old enough to have been warn in, and it has great market staying power because it is backed by so many manufacturers (Motorola, HTC and Samsung, to name a few), not to mention it's made by Google. Plus it has a huge app market, so you're constantly able to find new improved software to simplify life (not to mention games - a plus for even the most business oriented among us). Take a look, you may like what you see.