In my head better players will be more valuable on the flank. I feel like the level of play will be more equal with fixed economies and post-imp play, so having lower rated players reach that position will have them performing relatively better than when they get slaugthered at the flank by a better player.
Might not work like that in practice of course, maybe a better player will have a much better boom and can capitalize on that early imp. But if you assume flank plays very aggresive and full feudal and castle, while pocket booms and preps for post imp, I'd rather have the better players at the front.
Probably the way to think about it is whether it is a 3v3 or 4v4 because the macro skill (I mean game reading not ability to spam villagers) of the pocket matters more when they are structurally required to make decisions about which side to protect. But I can't imagine there is any hard or fast rule about which player to put where based on one factor alone.
I think it depends on the exact team composition, the strength of the lower tier players, and civ choices as well as the interactions between the players. Some weaker players might have a strong boom, others might be comparatively better at heavy flank aggression. Similarly, some of the higher tier players will be better than others in supporting their team with decision making and adapting to the other peoples play styles and strengths
Take this as just an anecdote, but I feel that on average it is best to have the stronger player as pocket, at least in the mid to low elo ranges, although typically the lower level players seem to hate being flank and prefer being pocket themselves.
I am ~1.3k RM ELO, and I play team games only with my friends, which range between 1.1k and 900 ELO.
Looking at the aoenexus stats (great website), when I play pocket our win rate is 5% more than when I play flank.
The above align with my personal feeling. I feel more in control of the game when playing pocket, as I am more relevant in the later stages of the game and it is easier to help the area of the map which requires more assistance.
I cannot tell however how this flank-pocket relationship changes as average ELO of the team increases.
I think the pocket is a more important position than flank since it can have more impact on the game due to its enhanced economic strength compared to the flank players. It would therefore would make sense to put the best player in pocket. However, I also think pocket is easier to play than flank. I think this is especially noticeable on very high levels where a 2300 rated pocket player is basically just as strong as a 2400 rated player.
So my conclusion is that at the top 50 level, it's better to have the stronger player in flank. Below that level, it is better to have the stronger player in pocket.