Why not offer both? I have both on my menu, and as they are such different services I wouldnt want to be without either of them.
The Solar mani is my standard mani treatment which is ace for dry or rough skin. It doesnt take as long as the spa, but its a very nice intensive moisturising and conditioning treatment. You can really market this one toward the ladies that love the idea of natural ingredients, since thats what the Solar range pretty much is!
The Spa is my deluxe mani. Its a nice citrusy summery scent, ideal for ladies going on their hols. Its more involved than the Solar, with a heat treatment and a citrus crystal exfoliation followed by an AHA treatment to really break down the outer dead skin layers, I describe this treatment as a rejuvinating treatment, you really can see the difference straight away after one treatment. I recommend this treatment for those that want their hands made to look as good as new and completely made-over.
I would seriously consider offering both, I get equal amounts of requests for both, and its nice to offer as wide a range of treatments to your natural nail clients as you do for your enhancement clients.