Of the two, April is likely to be best. Be prepared for some colder weather, and we often get a good dump of snow around that time. The up-side of this is that the hills will be rather resplendent with a good layer of snow either newly fallen, or retained from the winter months. If you can stretch it to late April, things just get better.
July is the start of the (Scottish) school holidays, so the whole place gets a bit busier. It can also be a bit wet, and the midge will be in full flow.
Your other option is late September/October, when we often get a spell of settled, warm weather.