If you specialise in Tax you will probably earn a bit mroe than a normal accountant
I am guessing with the Big 4 they will make you do ACA and then CTA after - so you will still be pretty well rounded.
Tax should be a harder but in someways it might be easier as you can then obviously just specialise in that area.
I would go for ACA and then CTA route so you have soemthing to fall back on if you do not liek tax - if you dod the normal ACA route you may get the option to specialse at the end of your training contract anyway.
Oh, and the CTA exams are emant to be mega hard!