Can SQL Server use a spatial index and another index at the same time and be efficient?
mickyt: With only 2 models in your dataset the spatial index will be more effective than an index on model id.  It will still do an index seek on the model id if required.  However if you start adding more and more models to the table, you may get to a tipping point.

I tested on a table with 1,000,000 points and 99 models.  Using the model index only was quicker and more efficient than forcing the spatial.  With 9 models over the 1,000,000, forcing the spatial index was quicker and more efficient.