to answer Riverguide's question...
you definitely do more boots and utes PT at Seniors. At least I did. They might have changed Juniors from when I was there last (02) but I definitely did more boots and utes PT at Seniors. For example, the only graded PT events you have in boots and utes in Juniors are the O course and the Stamina Course. At Seniors you have the O course, E course, CRT, Tarzan, more pugil sticks, more martial arts, and some other stuff I probably forgot. Plus you spend a lot more time in the field.
Another good item to stock up on for anyone headed to OCS for the first/second/third/whatever time is Blister Guard socks. They issue you 2 pair; arriving with 8 pair would not be overkill, since 8 pair will turn into 3 pair by week 6.
And definitely show up to seniors with all your stuff from Juniors. Even if it is "unservicable" (highly unlikely) you can use it for PT, or if it's really torn up, you can still use it for the requisite "2 trouser, 1 blouse" they make you pack for the humps.