IMO, Tate was drafted primarily as a KR and did a pretty decent job (in, effectively, his rookie season) contributing as the #4WR (most often as the token "deep guy"), and really as the #6 or #7 passing target. Edelman is a reserve slot guy who has developed into a very good PR
(by Pats historical standards, anyway) and can fill in as the #5 WR/slot guy. So, they're both "good to stay" as far as I'm concerned, for their overall contributions.
So, for me, questions wrt WR come down to Welker, Branch and Price. Including all pass-catchers, it comes down to Hernandez, Gronk and Woody as well.
Price is a complete unknown at this point.
Welker had been the premier slot guy in the NFL (maybe even in the history of the league) for three consecutive seasons. He had a down year (by HIS lofty standards) in 2010, probably attributable to his injury comeback. However, he is limited wrt what areas of the field he works to optimum effectiveness - namely, short/intermediate and between the numbers.
Branch can work the same areas as Welker (not nearly as well, but that's not as critical as it sounds). However, he also works very well outside the numbers, better than pretty much every other pass-catcher on the roster. He just isn't extremely effective beyond the intermediate zone.
Ultimately, Hernandez, Gronk and Woody all work the same areas as Welker with some variations that can create mis-matches (Gronk, Hernandez) or at least incur some defensive pre-snap decision-making based on where they line up.
But, therein is the problem, as I see it. With most of our pass-catchers, the defense knows roughly where they're going to end up. So, the passing game, in 2010, was somewhat predictable.
So, for me, if we're going to add a WR, height and/or speed wouldn't matter to me quite as much as "unpredictability" - IOW, I'd like a guy who can run ALL the routes (inside/outside, long/short/middle) with more or less equal effectiveness. The Packers receivers are all pretty much like that - each might end up anywhere, anytime.
Now, Price MAY be that guy no way to know, at this point. It's also possible that, in their second seasons, Gronk and Hernandez start working outside the numbers more often with greater effectiveness - which might actually add more tactical effectiveness to the passing game than a new guy who can only go long (since that's pretty predictable, too - see: Randy Moss). It's also possible that Tate, in his second year, can become fairly proficient running underneath routes and/or quick outs (which would make HIM that much less "predictable").
So, as to "who gets bumped off the roster" for a new WR:
Price - if the coaches have determined (unbeknownst to us) that he's a "bust".
Branch - if the new guy can run the same routes but is significantly faster and/or taller.
Welker - if the new guy can very effectively work the edges and deep and is decent over the middle, almost regardless of height or speed (I said, "almost" regardless).