I just built a V8, manual Mustang and Camaro to see how they stack up.
2015 Mustang GT:
Base GT MSRP - $32,300
Destination - $825
GT Performance Pack - $2,495
RECARO Seats - $1,595
Total - $37,215
435hp/400 lb/ft
The GT Performance pack includes a strut tower brace, larger radiator, unique chasis tuning, larger rear sway bar, deck spoiler delete, K-brace, heavy-duty front springs, Brembo 6 piston front brakes, 3.73 Torsen rear axle, oil pressure and vacuum gauges, unique stability control, EPAS and ABS tuning, and 19" black aluminum wheels. Well worth the $2500.
2015 Camaro SS:
1SS MSRP - $33,505
Destination - $995
V8 Coupe Performance Pack - $2,995
RECARO Seats - $1,995
Total - $39,490
426hp/420 lb/ft
The V8 Coupe Performance pack includes a black painted engine cover, strut tower brace, performance decals/sill plates and the 1LE performance suspension.
Those would be the most evenly matched options but no one in their right mind should ever pick the V8 Coupe Performance pack. For $500 more, you can get the 1LE package which includes not only the same performance suspension but also has 20 inch wheels, Goodyear Eagle Supercar tires, 3.91 final drive gear, and the ZL1 steering wheel and short shifter.
2015 Camaro SS 1LE:
1SS MSRP - $33,505
Destination - $995
1LE Performance Pack - $3,495
RECARO Seats - $1,995
Total - $39,990
In summary, I would take a 2015 Mustang GT over a 2015 Camaro SS but I would take a Camaro SS 1LE over the Mustang. Is the 1LE worth $3,000 more than a GT with the performance pack though? The 2015 Camaro is already much more expensive than the Mustang so the price gap once the gen 6 Camaro hits should be even greater.
(Edit) Vehicle estimated curb weights:
2015 Mustang GT, manual - 3,705 lb.
2015 Camaro SS, manual - 3,908 lb.