I went from a 2004 STX RCSB 4x2 to a 2014 5.0 SCREW 4x4. I couldn't believe how much had changed in 10 years. The engine felt twice as powerful, yet got much better gas mileage. My 2004 was lucky to get 16-16.5 hwy while the 2014 gets 16 in town (and I'm not easy on the accelerator in Atlanta). The best mod IMO is a tuner. My only complaint is Sync. It only works correctly 60% of the time.
2014 Ford F150 XLT SCrew 5.0L 4x4
2011 Jeep Grand Cherokee Limited 5.7L
2001 BMW 325Ci Convertible 2.5L
In all honesty the 5.0 is a great all round engine choice. Its get's just as good fuel economy as the 3.5TT and probably isn't much worse on gas than the base 3.7L. It won't tow like the 3.5TT but holds its own in a big way when you hook something up behind it.
I bought a 5.0 brand new in 2011 and had zero issues, traded it on a brand new 2013 and once again have had zero issues. Love the engine and I sit around 19mpg lifetime with my truck (see sig for specs), that's around 70% highway driving. Since the 5.0 is in its 4th year of production I'm sure the 2014 trucks will have very few issues and remain solid for many years. I'm hoping my 2013 lasts me 10+ years (knock on wood).
The 5.0 in the 2015 will be a beast, 25 more hp and 7 more ftlb of torque in a lighter platform will definitely increase the fun factor, but I wouldn't buy a 2015, I'd be waiting until 2017 most likely once the new transmissions have been out for a year (hopefully) and most of the bugs have been worked out. If I had to buy a new truck right now I'd be doing what you are doing and getting into a 2014 before they are all gone!
As far as mods are concerned all I have on mine is a K&N 63 series CAI, doesn't do much for power of fuel economy but sure does sound awesome when I put the fuel to it!