Ok, here's what I know now:
'Configurable Products' is for Product variants: Size, color, gender and so forth. So if the product is T-shirts, you can have 40 different shirt variants in stock, of various sizes and colours. So you make 40 Simple Products; 'Configurable Products' then lets the customer choose a Blue, 40in, men's shirt, using drop-down menus.
My problem is that there are 1000+ product options, in terms of spectacle-prescription values alone.
This example might make it clearer: http://theopticshop.co.uk/bench-glasses-bch-168.html
So with a configurable product I would have to add 1000+ Simple Products(?) Yikes!
So I still think 'Custom Options' is the way to go.
It would just be nice if a 'Prescription' could be a separate product that the customer was forced to buy. Then you wouldn't be filling up the database with 1000+options per product.