Janssen is 5 years younger than Sissoko. He has potential, quality and heart - none of which Sissoko does. Sissoko was an embarrassing 10 to 2 call who needs shipping to China asap.
Personally I just think Carroll was just scared to leave, and probably too nice a bloke/been part of the furniture so for long to push out.
Neither Janssen nor Sissoko should be judged on what they can contribute this season. Nevertheless, if you insist - Janssen's looked beyond woeful in many of his appearances, has struggled ridiculously to get up to speed, and his heart, while admirable, hasn't helped him look much better than Soldado did when he was in the same sort of form here. Sissoko's looked steadily better in each game he's played, has contributed multiple assists recently, looks dangerous when running at his man (while Janssen struggles to look dangerous at all, against top-flight opposition or against mid-table Championship side Villa) and has the physical tools needed to thrive in this league (strength, pace, size, etcetera).
Janssen in 5 years younger than Sissoko, yes. But on the evidence of how the two have played so far, If I were forced to evaluate the two...I'd say it's Janssen who looks like the embarassing 10 to 2 buy, not Sissoko. Again, I'd ideally write them both off this season and give them proper look throughout 2017/2018 - but it's clear that, in the minds of an awful lot of Spurs fans (yourself included), they're not starting on anything like an even slate. Sissoko's being dismissed because of his history, and Janssen's being persevered with despite being utterly crap (I'm sorry to say). Sissoko's performed better than Janssen has ,yet he's the one who needs shipping off to China while Janssen is given more chances?
If you're going to go down that route, at least be consistent and say that they should both be flogged to Shenzhen or wherever. My stance is that both of them have nothing to prove this season - your stance is that Janssen is worth persevering with over Sissoko despite him being leagues worse in his appearances so far. One is logically consistent...one just *isn't*. Maybe Sissoko goes, maybe he stays and Janssen gets shipped off back to AZ or somewhere, but that sort of predetermined bias against one of our signings based on his history (because there's really nothing else that would lead one to conclude that he's done worse than Janssen) isn't what we should be sticking with. We have to get beyond that and give everyone a fair crack of the whip.
As for Carroll, he was confident enough to go to Swansea on loan and quietly impress there, to the point where they would likely have bid for him if they hadn't undergone such rapid managerial changes over the past couple of seasons. I don't think him being scared to leave was a factor in his stay here - I think Poch wanted him around, and Carroll acquiesced as long as there was a mutual understanding that he'd play if he proved good enough. He hasn't proven good enough to displace Dembele/Wanyama/Dier et al, so he's chosen to leave - and I don't think Poch is really the type to hang on to players for sentimentality's sake.