I personally wouldn't be worried about sitting in the upper tier. I've been going to multiple sessions every year since the WTFs came to London, and perversely, in some ways I prefer sitting high up. At Wimbledon there's nothing better than being up close to the action (especially on the outside courts), but at the o2 I'm fairly indifferent.
Here are a couple of crappy photos from my phone from last year:
Upper tier (up in a corner) for Fedal (Tuesday night?)
Lower tier (pretty much courtside) for Djok v Ferrer (Wednesday night?)
I think the view from the top tier is fine as long as your eyesight in reasonably good. For the night sessions I also think the atmosphere is better up there; in my experience lower tier courtside is largely populated by City workers on hospitality who arrive late and drunk, leave early, talk throughout the matches, and have at best only a passing interest in tennis (i.e spend the evening shouting supposedly ironic rubbish while guffawing).
It's a great event though. As you're on holiday (and with your wife), then it's advisable to buy tickets in advance, but for the past two years I have always been able to pick up much cheaper tickets a day or so before from online agents (Viagogo / Ticketmaster - but only for upper tier 'unspecified' seats) if I decide I want to go to an extra session.
Have a great time!