What would be the deciding factor for you to go with or without the tarp? Weather, site selection, etc.?
Both of those bivy's are very robust. With being robust comes weight and $$$.
Let's say your shelter system choice is based on weather. In this case, you bring a tarp and have adequate protection from rain, snow, etc. This would, theoretically, negate the need for an eVent top, saving weight and money. On nights where there is no call for precip, you could just break out the bivy. This is all speculation on my part, as I'm not sure this is your intended plan.
If your decision on tarp or no tarp is based on site selection (i.e. - not be able to find a spot large enough to pitch a tarp), then your standalone bivy should be robust.
For me, I've had great success, in perfect and nasty weather, with a solo sized tarp and water resistant bivy.
If I could throw other options out there, I would say take a look at the MLD SuperLight and the TiGoat Raven bivys. The new Superlight will fit your NeoAir, and DJ at TiGoat is working on a new Raven bivy that will fit it (but, call him... you'll have to give him specifics as the current version on the website would not fit a Neoair).
If I had to cast a vote, I would vote for the Soul Side Zip as I've seen one of these, and they are really sweet. Plus the netting gives more venting/warm weather functionality.
Let us know what you go with and good luck!