Well it depends doesn't it. I have an Exec but had the Premier for the weekend before I got it. Firstly the money, £168 is nothing for a car if that includes all your maintenance etc. which I assume it will seeing as it is a company car. If you can afford it (especially if you were paying something like that before) then I would go for it. For me the price difference was just too much between the two but if I could have afforded it I would have.
As far as the actual equipment goes you are getting a lot more for your money. The premium nav comes with better traffic, online connectivity (although I question the real value of that other than traffic), the reversing camera and parking assist which I found was bloody good on the CT I had previously, better integration than the exec, voice control across many of the car functions like picking your track on your iPhone, having your texts read out to you, having the nav show on the smaller instrument display, splitting the main screen to show different functions and so on. Factor in all the additional safety equipment (which should be standard nowadays) especially with the blind spot for the motorways etc. and I think it is good value.
You are spending a long time in the car and you want to be happy there. The seats are more adjustable and ventilated as well as having the easy entry system although make sure you don't have an adult behind you when you undo the seatbelt! Unlike DJP I listen to Spotify all the time and I thought the ML sound system was fantastic - it even made the Frozen soundtrack sound good! I can imagine some not liking the way it splits your music across the various channels but I would have it in an instant. Having said that - the standard sound system is actually pretty decent and just as good as the upgraded sound pack that was in the CT although it can't do surround sound of course. I would go to the dealer and have a listen.
If comfort is your main priority though what about the new advance trim. Effectively the Exec with the heated/vented seats and parking camera? Would be a good compromise as it still has all you need and will have the great ride you get on the 17s. Apart from looks, the Premier doesn't gain anything from the bigger wheels and wider tyres..
I wouldn't put you off the Exec by the way. It is a great car and the nav is simple enough to use as well as the rest of the bits with the rotary dial rather than the remote touch. It is just a bit more simple from a tech perspective.
I don't know if any of that helps but it will be interesting to hear what you go for!