Oh wow, sorry for all the massive double posts >_>. This frigging site is being slow and weird as hell today.
@Luvtospooge - Demon's ruins are definetely the hardest choice of the three, and the most worthless since you won't be able to progress much further in, due to a barrier blocking your way that will only be lifted once you receive the Lordvessel.
Sen's fortress is the way to go if you want to continue the story, but it's a mid-high level area if you're going there right from ringing the bells. Your best option will be to continue exploring optional areas. There are three invisible enemies in the Darkroot Garden, although only the thief is the permanent one. Once you beat the two-handed sword knight and the bow invisible enemy, they'll be permanently dead.
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"
