Just to add my 5 penneth.
Currently, whether you join Mainline, or Tfly, there is only ONE seniority list for Britannia pilots. Also, under current Ts & Cs, Tfly pilots have the option to bid for a mainline position after 5 yrs' service & enjoy mainline 'perks' (salary, pension, holidays etc, etc).
HOWEVER, last week's announcement has currently raised more questions than there are answers; hopefully, we will find out more about the proposals/negotations soon.
There is also the possibility in the future of 737/757 shared type ratings - something that would be an advantage at DSA - but my guess is that this is still some way off for a variety of reasons (CAA approval, BALPA etc).
I agree with Max that the Ts & Cs of Tfly are likely to be raised for the reasons he states, but I can envisage some reduction in mainline Ts & Cs as well. How much & who will be affected, I just don't know at the moment.
Whatever you choose, good luck & all the best.
Backtrack.