Here's a list of advantages/disadvantages the Thunderbolt has over the Bionic:
Thunderbolt has more RAM: 756 compared to Bionic's 512.
Thunderbolt has a Kickstand - and contrary to popular belief, you CAN charge it while using it, because the Kickstand also lets you stand the phone upright.
Thunderbolt has a 32GB microSD card pre-installed, Bionic doesn't have one.
Thunderbolt has Dual-LED flash, Bionic has single.
Thunderbolt can handle data and voice over 3G/4G, Bionic can't; What that means is you're able to say, Google something while in the middle of a phone call.
Thunderbolt has a better FFC (1.3MP), Bionic has 0.3 FFC.
Thunderbolt has a built-in FM radio (Not sure if there's an app for this, but hey).
Thunderbolt has an external speaker; Bionic doesn't.
Thunderbolt has DLNA connectivity, Bionic doesn't (to my knowledge)
Thunderbolt will have an unlocked bootloader, Bionic will almost certainly have a locked one.
Thunderbolt's back facing camera supposedly has a better lens than the Bionic (/Inc./EVO)
Sense 2.0, while a majority of people prefer it to MotoBlur, it's a matter of preference.
Thunderbolt will be released sometime in February (the 14th, 24th, 28th are some speculated release dates), While BIONIC is prepped for a release in the 2nd quarter between April and June
Thunderbolt will have a device that allows connecting to non-DLNA enabled TV's, while the Bionic can only be connected to HDMI-enabled TV's.
The Thunderbolt has a sleeker design than the Bionic (A matter of opinion, but mostly agreed upon.
DROID BIONIC Advantages over Thunderbolt:
Dual-Core Tegra 2 Processor, both clocked at 1GHz (Although according to this video: , the Thunderbolt is still faster).
DDR2 RAM instead of regular (I think DDR2 RAM reads/writes faster).
Higher resolution, 540 by 960 for a total of 256 pixels per square inch, compared to the Thunderbolt's 480 by 800 for a total of 217 pixels per square inch).
Better battery (1930 mAh compared to the Thunderbolt's 1400 mAh)
More built-in memory (16GB's compared to the Thunderbolt's 8GB's, although the BIONIC doesn't have a microSD pre-installed, so the Thunderbolt will still have more memory out of the box at 40GBs).
Bionic will have more accessories at launch.
If I missed anything or made a mistake, post it and I'll add/change it.