Both guitars are good for the type of songs they are meant for. Because they have different types of pickups, Epi's humbuckers and the Strat's single coils, they will either lend a fat (Epi) sound or a brighter twang to the song. So it all depends on the songs you play and how you want them to sound.
Keep the Epi and get the Fender Strat for variety. Further down the road, you can get a Tele, a Floyd Rose-type guitar, a semi-hollow, a hollow body with P90s, an acoustic, a classical and a bass to finish your collection.
the strat simply can't substitute the epi, vice versa. if i were you, i'd invest in a single coil guitar as i already have a humbucking option. the strat also represents a totally different feel (unfinished neck/ longer scale length etc...) & this might trigger other inspirations 8)