Off the top of my the Accord has a nicer interior, a smoother and quieter ride, it retains value better, and you can give it wide open throttle every single day and it would still need maintenance less often than a leisurely driven Mustang. Those are the primary reasons I would choose an Accord V6 over a Mustang. I wouldn't buy it expecting it to beat a Mustang in a straight line.
The Mustang is primarily a car for people with the budget to get a Civic but want something fast in a straight line. Don't overplay it, it's not a Hellcat, it's slightly quicker than the Accord V6.
Interiors on the EX-L v6 and Mustang Premium are similar. Some may like the design of one or the other better; however, the quality is similar. The 2015 mustang interior is much nicer than the previous model.
I would assume that the Accord does have a smoother ride as it has the suspension of a family sedan. I haven't seen any actual data on interior sound for either car so I don't know. I've driven a 2015 premium GT and it was pretty damn quiet inside. I was a little disappointed that I couldn't hear more of the engine/exhaust.
Regarding maintenance costs, Edmunds puts the mustang GT premium at $4,242 over 5 years with the accord EX-L at $3,946 over 5 years. So it's $300 cheaper over $5 years... Also - mustangs are NOT expensive to work on in general. Edmunds didn't have statistics for the ecoboost trims up as of today.
Regarding holding value, Edmunds puts the EX-L between the V6 mustang and mustang GT as far as depreciation is concerned. $13,179 over 5 years for the V6 stang, $15,875 for the EX-L coupe, $17,143 for the GT, and $18,946 for the GT premium. I'd be willing to bet that when the eco boost premium figures are up it slots in VERY close to the EX-L coupe given their very similar base prices.
The problem is you described some GREAT traits of the Accord sedan (which is a great family sedan). When you go to the coupe version you make all the normal coupe sacrifices around not really having a functional back seat and gain NOTHING. You basically are just doing it for looks.
The Accord coupe is primarily a car for people that previously owned a Civic and now have a [slightly] larger budget and want something faster in a straight line but are too "cool" to buy the Accord sedan (or a true sports coupe from a number of different manufacturers). Don't overplay it, it's not a sports car, it's slightly quicker than the Civic Si but much less of a "sports coupe."