And I want to talk about both, in the context about what we know about the Marvel Cinematic Universe. So. Spoilers?
Agents of S.H.I.E.L.D. has not been the most consistently entertaining show ever made, even though the show's first season has had some interesting plot threads. Of these threads, one has been fairly interesting (why is Phil Coulson alive, instead of dead?), one has been not as beloved (what is Skye if not human?), and one has been all-over-the-place (who on the Bus can be trusted?). The one that io9/the ODeck/the fandom appears to love almost without reserve is one that had to wait for a bigger part of the MCU to happen before it could be "allowed" to occur: that S.H.I.E.L.D., for decades, has been a front for Hydra, and not everyone in S.H.I.E.L.D. is actually loyal to it. This event also allowed another major plot thread (who is the Clairvoyant?) to finally be resolved. Well, mostly (tune in next Tuesday for the season finale!).
The pacing of season 1 had to deal with the constraint of being a small cog in the MCU. On one hand, that's excellent work on Marvel's part of building a self-contained universe that's not full of low-quality product. On the other hand, Agents of S.H.I.E.L.D. had to wait until its 17th episode of its 22 episode season to finally talk about Hydra. This gave its undercover agent 16 episodes to worm his way into our hearts, which is good because he's good at worming. But the downside is that his character was among the least-fleshed-out by that time on the show; other characters (particularly Skye) had had their own histories covered in some detail, whereas the turncoat was mostly an enigma.
In the comics world, Event and Crossover issues, particularly over the past decade or so, have had their sales enhanced by being promoted ahead of time. Readers who might not be overly-interested in Captain America might care to read an issue about his death, and Marvel smartly recognizes that telling everyone about this plot twist might be a good idea for their sales. Similarly, viewers might have been interested in knowing that Agents of S.H.I.E.L.D. was going to tie in with the MCU in a big way... Except that the show couldn't do much to promote that, because it had already played that card before by having two Thor-advertised episodes that had not a lot to do with Thor, and audiences were sick of the show doing that.
The good news is that the show doesn't have to hide in that shadow anymore. Guardians of the Galaxy will have developments that affect the rest of the MCU, particularly in establishing more connections between Earth, earthlings, and the more Cosmic elements (Thanos!!!!) of the Marvel Universe that the MCU has access to (sorry, Silver Surfer, you're owned by Fox). GotG will happen before the second season of Agents, so Agents can add in those new connections.
Age of Ultron will hit theatres right in the area of episode 20 or 21 of Agents's second season. That ... could work out for the best for everyone. Agents could be a place wherein the defunct S.H.I.E.L.D. gets to, at least partially, reorganize for the benefit of assembling the Avengers again. Or maybe it can deal with AI, so that Ultron's origins can be talked about. Or the show can continue to do what it's been doing greatly: playing up its angle of showing that Hydra is very very interested in creating its own, to steal the wording from Baron Von Strucker, "miracles", in order to match and/or beat the heroes that the Avengers Initiative brought together.
As a "prequel" of sorts, Agent Carter gains all the benefits of being part of the MCU without having to deal with the drawbacks of competing with the movies for plot-sensitive information. This is a win-win for the show, as it gets to establish in the MCU how and where S.H.I.E.L.D. was formed, how Hydra got its sneaky tentacles into S.H.I.E.L.D., how the Starks are involved with S.H.I.E.L.D., etc. If it's a year-long show (which, come on ABC, make it so), it might also have a few things to say about Age of Ultron, particularly because Agent Carter is appearing in Age of Ultron, although it's currently unknown if that will be 1940s Carter or not.
Overall, I'm really interested in seeing how these tv shows are able to develop on their own and in relation to the MCU they're part of. Best of luck to both of them and, of course