Is your plan for the individual versions to be subpar if considered as stand alone or do you intend for them to be on par?
I think modal cards are a good idea and I think using the card slot as the deciding factor was a wise choice.
Does Graviton Enforcer get the +1|+1 or just gain the momentum ability/status?
In the first case I would reduce the hp by 1 or increase the casting cost/activation cost by 1
to make it at par.
In the second case I would reduce the cost by 1
to make it at par.
I would think Gravity Pull would not be not very useful on Graviton Protector.
As such I would reduce the cost by 1
to make it on par.
Card Text that would fit.:
"Create either a Graviton Enforcer or a Graviton Protector depending on the creature slot."