Thanks for your post, Dave T. I agree that, while Ben is right that the tablets are more potent, my experience has never been that they are absolutely necessary. I've used Polar Pure, bleach, AM drops, and AM tablets at one time or another, and I've never had any intestinal bugs on long backpacking trips. Except once when I didn't treat my water, but that went away by itself in a couple weeks :)
At least five people that I hiked the Appalachian Trail with never once treated their water, and never had any issues, either. I've met one hiker who did have giardia, and one hiker who told stories of getting it several years ago, but the vast majority of hikers, whether they treat water or not, never get it.
Of course, water sources on the AT and PCT are pretty different from what I hear, so here's my plan. I'm taking AM drops, but I've also got a pile of tablets that I have left over from last fall. I'll carry 5 to 10 tablets with me, at very little weight cost, and keep the rest in my bounce box. Use drops on water sources that look fine, tablets on those that look less fine. And in general, not worry about what's in the water once I treat it.