I own a Droid and have spent extensive time using the Pre Plus. The Droid pretty much trounces the Pre in hardware but WebOS has some definite advantages over Android for UI.
Here's a quick summary of how the devices compare:
Droid Pros:
-Better camera & video quality (though MUCH slower in opening the camera app & focusing then the Pre)
-Better voice quality & reception (marginally, but nowhere near as good as older Moto dumbphones were)
-MicroSDHC slot. 16gb included & expandable. A huge positive over the Pre
-Better battery life & overall performance provided you aren't running a ton of crap in the background on your Droid.
-Vastly superior build quality and slider response
-Bigger, higher-resolution screen. This is arguably the one that will seal the deal for many for the Droid.
-Many more apps + free apps + overall developer/community support for Android vs. WebOS
-"Free" Google Maps navigation on the Droid. This is countered by the free wi-fi hotspot on the Pre Plus. I wouldn't be surprised if VZW pulls the free hotspot service at any time and I think the navigation is more useful on a day to day basis.
-Better media capabilities on the Droid but both devices still pale in comparison to an iPhone.
Pre Pros:
-Better PIMs. Not as good as the old Palm OS PIMs but still better than the rubbish that Google puts in Android.
-Better keyboard
-Formfactor in general is more pocketable & comfortable to hold
-Better universal search/Synergy
-WebOS has much better multitasking & memory management than Android
-Free wi-fi hotspot feature. See my comments above.
-More RAM (512mb) vs the Droid (256mb) and the internal storage is fixed but faster than a slow Class 2 miroSDHC card.
-Touchstone. Best accessory release of last year IMO.
Late last year I was desperate for a new VZW smartphone that wasn't a BB or WinMob. I would've bought the original 8GB Pre had it been out on Verizon before the Droid. But the Pre Plus didn't come until late January of this year, so I got a Droid on launch day. I still feel that both Android and WebOS have tons of shortcomings but the next 6-12 months should see both platforms improving considerably. For right now, I'd get the Droid if you can.