If you want the convenience of not having to do the upgrade yourself, and can settle with the stock
SSD, then configure the system with the biggest drive you can afford when you order from Dell. However, the Runcore
SSD from MyDigitalDiscount is a bit faster than the stock drive, but you would have to install it yourself (super easy). I'm not sure what the latest price differences are between Runcore and stock drives are.
A lot of people tend to order their Minis with the smallest possible
SSD (cheapest configuration), then upgrade after receiving it. I believe there might be one or two other brands of
SSD that are compatible with the Mini, but from what I've seen on the forums, the speeds might not be optimal.