Pictures would help.. Some of us are more visual.

Sounds like it is largely a matter of taste. Keep in mind what the maximum height of your plantings will be and whatever technical requirements the area has.
I don't have a need to use the same species on each side but some people love that symmetry. However, to use the same species on either side would be difficult if you want the plants on one side to be taller than the other. However, you could use plants of similar colors, etc. that reach different maximum heights. For instance, small evergreen species on one side and larger evergreen species on the other. I suppose you could also go with dwarf varieties on one side and standard varieties on the other.

imo there should be some symmetry. here's how I would think about this: create layers (tall, med, small), but vary the intensity (depth) of each layer. i.e. on the lower portion have an abundance of small (so it might look like they "pooled" there). i would try not to make it too top heavy. leave room for an accent plant somewhere. somehow leave room for gardening tasks, like prunning, weeding & watering