How old is Junior? I'd recommend a Wii if he is under 12, it's family fun system and there are a lot of great titles for that younger age range. But I'm guessing you are looking for a more beefy system, so...
I've owned both the 360 and the PS3. I loved them both. But I would steer you towards the PS3 at this time for a few reasons.
1. Do you own a decent gaming PC? If you do, most 360 titles are also available on the PC.
2. Hardware failures are terribly common for the 360. I went through 3 systems. Everyone here seems to have lost at least one 360 to failure.
3. Blu-Ray player is on the PS3. You are getting a new TV, and it will be nice to have the option to play Blu-Ray discs. It really looks fantastic.
4. PS3 is slightly better hardware, so the lifespan of the system will probably be longer. If you want to go with one console this is a big plus. Heck, the PS2 still has games coming out on it.
5. The PS3 game catalogue is just starting to hit its stride, whereas the 360 doesn't have too many exclusives in the pipeline that aren't on the PC as well.
6. I think they just cut the price, so it is not really much more that than the 360.
The weakest link on the PS3 is the multiplayer support/community. It's not terrible, but the 360 is fantastic and has a lot of built in neat stuff like Netflix (which is actually available on the PS3 but requires some software.) So if you want to play online with friends, it really comes down to what your friends have.
There isn't a bad decision here though, all three consoles are very solid products. Enjoy!