Fantasy wise, I don't know if we can really consider anyone north of 35 "reliable."
Brady and Manning are both solid NFL QBs, and fantasy wise they've been QB1s for some time when not injured. Brady had a little slouch last year, and ended up 13th, but his run from week 9 to week 15 showed that he could still be a dominant fantasy QB, and being 8th this year keeps that train running.
The thing with projecting out a year for old guys is that we don't know what happens in the interim.
If Peyton wins a Superbowl, does he retire? Does Brady if he wins one?
If neither wins one, do they remain healthy? They're climbing towards 40, but health can be sneaky, they might look fine and then get injured mid-season and that's it.
The best thing to do with guys like Manning and Brady is to watch them closely in the offseason, see how they're doing, and what their health is like.
If everything checks out there pick them in their usual spot (high QB1 for Manning, low QB1 for Brady), BUT then you need to pick their backup a little earlier. Make sure whoever is waiting in the wings is a good enough fantasy player to step in (so, to answer the question at the top, I'd rate Peyton as slightly more reliable, but that just from his usual fantasy performances).
For example, if you grabbed Peyton, and are in a 12 team league, you should be picking your backup early enough that you get the 13th or 14th ranked fantasy QB, right below guys who would be considered starters, and that probably means picking your backup earlier than just about everyone else you're drafting with.
And of course, watch the waiver wire closely in the early weeks. If a QB looks good, like he'll be fantasy starter material but is riding the waiver wire, snatch him up too, though how often you'll find guys like that I don't know.
Dealing with older top tier fantasy players is all about hoping they maintain through the season while prepping for the inevitable age drop off that might happen on your fantasy watch.