I have titebond 2 and 3. The only difference between 2 and 3 is its waterproof ability. TB2 is described as "Water resistant". While TB3 is described as "Completly waterproof" and best used for "Marine apilications". Also, TB3 dries a MUCH darker color than TB2. It turns a very dark brown.
For modeling purposes I dont think theres an advantage to ethier one... unless that is you plan on getting you're plane very wet
