I have both of them.. while the TF300 is nice, the N7 has a better screen and is marginally faster. The Tegra3 in the TF300 is 1.2ghz, whereas the N7 I believe is 1.3ghz
The N7 is definitely more portable and easier for reading books and playing games.
The N7 is also on Google's update path, whereas the TF300 is on Asus' that means you'll get Android updates sooner on the N7.
Here's a good versus link,
Nexus 7 Details vs ASUS Transformer Pad TF300 Details in Tablets