Originally Posted by TRAVELSIG

Remember that on these flights business class is an economy seat with the middle seat blocked and on slightly better food with LH.

I would suggest you take the TK nonstop IST-MAD- I took this flight last year and it is far better than the connection in FRA. Also- TK is STAR alliance as well so you will still get your points/miles with UA or you can redeem also with UA for this ticket. Even if you end up in economy avoiding the connection is worth it.