I think if it was me the decision making process would go like this - my aim would be a permanent position, is that yours?
First of all are BY offering you a permanant position?
If yes then I would accept their offer as it would save you the worry of finding work for the winter if a permanent position is what you want. (Or perhaps you have work organised for the winter) Or are you more likely to get a permanent position sooner with BY than MYT?
If not, then if you were happy with MYT go back there, as you say you will be further up the seniority list. In the mean time keep looking around at other companies for full-time employment unless CWL is the only base you are considering then of course you are limited.
The other thing I've just thought of is that as alot of people as you say have left MYT I think it will feel like a different company anyway with a whole load of new people. So a change in company probably wouldn't be as radical as you think although companies can treat you very differently. Do you know anyone at BY you can ask?
I tend to go with my thoughts when I first wake up in the morning sometimes too. Not at all logical I know but they tend to be the right decisions when I go with my gut feeling. Heck does any of that help you? I'm more confused than you are now.
All the best.
D