Dispute it's totally irrelevant, team dynamics are important and do play their part. If a bloke is taking wickets against any Test side, I think you have to back him to do the same against another. Sidebottom had incumbency and that would need to be respected initially but after a while, if the unit is bowling well, I personally would be loathe to mess with that.
It's a really, really tough balancing act though because there are many exceptions. If the bloke coming back is a gun then the rest of the team would have an understanding that if fit, he's in. And, I guess, to answer the question of who is more likely to take wickets for you, I don't think it's so clear-cut that if Sidebottom is fit, Onions is out.
Oh, i totally agree, that's why I was asking. I saw Sidebottom bowl against Lancashire on the final day of a four-day game in May this year, and he didn't look like he'd ever be the same again. His pace was down, his line and his length were both defensive (unforgivable considering they were trying to force a result) and he just didn't look like taking wickets. Meanwhile, Graham Onions has been absolutely tearing the county circuit apart this season.
On the other hand, Sidebottom has an excellent test record in spite of his fitness issues and, on the whole, a much better FC record than Onions. Considering that whoever's going to be picked will be the side's fifth bowler, Sidebottom's control might be more desirable than Onions's attacking lengths that often leak runs. His left-armers give a little variation on what you already have and, if fully fit (and that's a big if), he's probably an all-round better bowler than Onions.
Who's going to take more wickets is most certainly a big call. I was disagreeing with the method of the previously quoted post, but not with the conclusion.
Regarding your point on team dynamics, I don't really agree with it. Playing an inferior bowler over a better one because he happened to be playing when the team was winning isn't a good idea, particularly when there's a big step up in quality. That's the method that saw Giles picked over Panesar last time round, and I don't think England should make a similar mistake this time. You play your best team.