Meganium-
Pros-
-Good defenses
-Wide/varied movepool
-Starter (Starters are more likely to receive more attention, which eventually leads them to be overlevelled- if you have a level cap this might be a problem, though)
-Well rounded stats
-Learns a decent amount of HMs
-Decent Speed
Cons-
-Meganium has the most weaknesses out of all the starters
-Meganium also has the most disadvantages with the Gyms (normally this wouldn't be a problem but it's your starter, soooo...)
Sunflora
Pros-
-Good SpA
-Variation between special and physical moves (this can be helpful in a nuzlocke if, say your Sunflora has an Adamant nature)
Cons-
-Horrible Speed
-Poor Defense
-Useless Abilities (both of its Abilities are based on sunlight, don't waste a move spot on Sunny Day- and I highly doubt you'll find a Hidden Ability one)
-Pretty difficult to evolve (You can only find the Sun Stone by winning a Bug Catching Contest or buying it in Pokeathlon Dome. You can also find it on Route 13 in Kanto. You want to evolve Sunkern as quickly as possible- it's weak enough to be infamous for it)
With all that said, you should probably pick Meganium. I normally wouldn't be too picky, but Sunkern is extremely weak unless you overlevel it like crazy. It could seriously affect your nuzlocke.
Hope I helped :)