Agree completely with the above comments. If you are really only looking to change to save the additional fuel cost, well as noted above, you can buy an awful lot of fuel for what it will cost you to change the car, and you'll get a car which is probably less well-suited to longer journeys.
If you want a change of car anyway, though, go for it and use 'saving money on fuel' as a means of doing the man-maths.
I know what I'd be doing though, and that's sprucing up the 159 a bit so it's even more comfy for spending hours in on the motorway. Fit HIDs for the night journeys, give the interior a really thorough cleanup, finding a nice insulated coffee cup that actually fits in the centre console etc, get a bracket so you can connect your iPhone up properly to the stereo. That sort of thing.