I have Motherwear "essential" dresses and tops, with the vertical slits, center opening and the demi-flap. I also have the linen tops that are pretty but huge and I hate ironing, they have the button flap at the waist level..not my favorite.
So I would go with the snap center and vertical slits for initial ease with little latchers. Snap center is great with the Double Blessings tandem nursing pillow - if you need to buy one, let me know.
When the twins were newborn, clothes required a lot more ability to latch them - so some of the slit styles would get in the way. I remember one visit to a relative's house where I used a binder clip to hold the center in place so I could latch them without fiddling with the material - very annoying.
The good side is if you are shy ( I generally have no modesty while nursing
) that some of them remain pretty covered, while almost unlatching them if you are tandem. Most of these tops are made for singletons to be completely comortable and do not genereally require having enough material to stretch.
I only wish that I had the Target top I have now, and used a blanket in certain social cases.
I think I will design a tandem top with coverage, now that you have inspired my imagination
I have written to Motherwear before about measurements of slits and material for tandem ease of newborns.
And after all this, it gets SO MUCH EASIER when they hold their heads up and latch by themselves as tandem pros.