Really helpful replies, thankyou.
As has been stated PGS is still coming up more than £100 cheaper including baggage, and actually for dates I'm looking at their fully flexible fare class (with plenty of baggage & food included) is over £100 cheaper than TK's standard fare !!

Comfort is a big consideration for me as I haven't done economy for anything over 3hrs for a long time (awful back & neck following a non-fault vehicle accident).

So....perhaps I should stump up the extra £100 this time around. I was wondering whether leg room and seat comfort were similar on pgs and TK but it sounds like TK is better in that regard.
Will check out Turkish Flyer on YouTube as suggested.

Not too worried about going into & out of IST or SAW but I guess IST is better placed for a day out in town. I'd find things to go see on the other side though anyway.

Don't have status with TK or an alliance partner so that doesn't factor in either, although I do have Platinum Plus for a lounge.

Thanks again everyone.