I'd also vote for TK. The lounge in IST beats the United Club in every way. TK seats are great for couples. I find them a bit close when I'm sitting with someone I don't know. Be sure to get the window bank of 2, instead of the centre bank of 3. IIRC, TK does seat reservations a few weeks in advance, not at the time of booking so you may need to call into their Canadian call centre a few weeks before the trip.

The food on TK is top notch, if that is a consideration.