Brown is out another 5-7 days with an injury, which means he likely won't be up for at least another few weeks. Its not serious but he won't be recalled immediately upon getting back into the lineup.
I would hold Jennings. I added him recently in a league and plan to do it in a few more tomorrow, but today was the absolute earliest he was possibly going to come up (and that would be inciting the wrath of the players union so I didn't necessarily expect it), and I think there's a great chance he comes up on Monday. Its a service time issue. If the Rays called him up before today, they would lose a year of service time. This isn't a Super-2 issue because he was already up last year. Unless Sam Fuld goes on another tear, there's no reason for them to wait the two extra months it would take to deprive him of Super-2. If Sam Fuld goes like, 7 for 10 over the weekend and there's no announcement that Jennings is coming up Monday, then I might be a bit worried, but I'm gambling that he'll be up by the end of next weekend and if you put the Over/Under as Friday, I'd take the under.
Also worth noting, Jennings has homered in back to back games for Durham, going 4 for 9 with one walk and one strikeout. He's now hitting .300 / .414 / .492 with five homers and eight steals. He also has 21 walks against 28 strikeouts in just about 160 PA's, which is roughly a league average strikeout rate and an outstanding walk rate. I think he has a higher floor than Brown, as long as the walks carry over to the majors, even if they come down a bit, there will be value in his legs. If the power and average carry over a bit too, he's at least a #3 OF in mixed leagues. I'm going to optimstically project him around .270-60-8-45-25.
For what its worth, the next major prospect called up after Jennings will be Brandon Belt, for similar reasons, though I wouldn't expect him before next weekend. My guess is Jennings is called up this Monday and Belt is called up Tuesday the 24th. It becomes a bit more of a crapshoot after that. Brown could be next, or Mike Moustakas. I don't think a guy like Anthony Rizzo, Brett Lawrie, or Dustin Ackley will be called up before at least one more major callup is made after Jennings and Belt (not counting Brown or Mike Minor, because in terms of service time they'll likely wind up in the previous class), since those teams don't have a whole lot to gain by gambling with their prospects service clocks.