This is a personal opinion and my advice is to choose the option that will fit your personal needs the best. Some people use their Minis differently than others... so more disk space may be more practical.
On the other hand, if you do not need your Mini to have a lot of storage space... then sticking with the default HDD or opting for a SSD
may be your best option. I personally "upgraded" my HDD to a 320Gb Western Digital... but I look back on the decision wondering why. lol.
In the end, I wish I would have gone the opposite direction and waited for "value SSDs" to come out that still have good controllers in them. Now that SSDs have been around for a while, we will start to see offerings that are more affordable.
But, like I said, if you are going to be storing a lot of data on your Mini and using it to carry around a lot of music, movies, documents, applications, multi-boot... then you want a larger drive (obviously). If you are like me, then you will not use your Mini to store many files because they are hosted on a home server... and the files you do need to have locally on your Mini will fit on the stock HDD or a SSD