I have a 225 and a Fenix 3. For road running and most workouts, I use the 225 because it's lighter and has the HR monitor.
The main thing for me, is I wear the Fenix everyday like a normal watch. It's nice because sometimes I forget to bring my 225 to work (I run at lunch) or for a run after work, plus I use it for biking, swimming and as my step counter/activity tracker for my work health benefit thing. So it doubles as an everyday watch, but has all the GPS stuff in case I want to do a run and don't have my 225 with me.
Also, as mentioned, it's great for ultra events. I did a 100K trail run last year and never had to worry about battery life. In general, if I use the Fenix 3 for all activities, I only have to charge it every ten days to two weeks. I am charging my 225 every other day if I use it for training. I take the Fenix when I travel, since I don't have to bring a charger either.
Battery life is great. Versatility is the other.. it covers everything I'll ever want to do.