The ability to replace the battery for 8-10 bucks rather than the whole unit if it an eGo unit fails. Also, you will eventually want a "top-end" mod if you are like most vapers, so this next one is your stepping stone mod. The eVic is a pretty mod with a nice display, but has a low amp limit and questionable durability. The Vamo is a popular, more durable mod with a very high amp limit and is pretty darn cheap.
I owned both of these mods and I can honestly say I preferred my eVic, but that was because the amp limit did not affect me with my vaping profile. Both mods are not particularly accurate on voltage output when the battery gets 50% or less.
I know nothing of the SVD and MVP, so look to others for advice on these.