That all depends on what you want.
OS
TF300 - ICS (nearly stock)
Galaxy Tab 2 10.1 - ICS with TouchWhiz
OS Updates
Asus has a track record of providing the fastest updates of any manufacturer.
Samsung is mediocre at best, although they've improved from being absolutely abysmal.
Processor
TF300 - Tegra 3 quad-core processor clocked at 1200MHz
Galaxy Tab 2 10.1 - dual-core processor clocked at 1000MHz
(and both have 1GB of RAM to go along with that)
Internal Memory (at $399)
TF300 - 32GB
Galaxy Tab 2 10.1 - 16GB
Expandable Memory
Both have a microSD slot.
Ports
TF300 - Proprietary, microHDMI, 3.5mm headphone jack
Galaxy Tab 2 10.1 - microUSB, 3.5mm headphone jack
Connectivity
TF300 - WiFi, Bluetooth 3.0, 3G/4G coming at an unanounced date
Galaxy Tab 2 10.1 - WiFi, Bluetooth 3.0, 3G/4G available at launch
Cameras
TF300 - 8MP rear with F2.2 aperture and back-lit sensor, 1080p video recording. 1.3MP front.
Galaxy Tab 2 10.1 - 3MP rear, 720p video recording. 0.3MP front.
Battery Life
No one can say for certain yet, but I'd venture to say the Galaxy Tab 2 10.1 might see marginally better battery life. But that's unknown at this point. The battery life on the TF300 is very good.
Screen Quality
Resolution is the same on both, but based on their previous products, the quality of the display is probably going to be a little better on the Galaxy Tab 2 10.1. Displays are something Samsung does very well.
Then of course, in the TF300's corner is the keyboard dock that not only adds a full keyboard and trackpad, but also a full size USB port, a full size SD card slot, and five hours of additional battery life. The Galaxy Tab 2 10.1 will have keyboards and other accessories, but none as innovative and useful as the keyboard docks the Transformer line has.
For me, it's the TF300 all the way. It has better specs (most notably the processor), better proven support, plus there's the sexy keyboard dock. The Galaxy Tab 2 10.1 just doesn't offer anything that's beyond the TF300 at the same price point. In fact, in most respects, it offers less. The screen and the battery life would have to absolutely blow the TF300 away for it to be a viable option in my opinion, and it's not likely to be all that much better in either of those.