I've done booth with mine over the years. The ones I've sealed stays nicer for longer. I use mine as a general working table (not only for cutting). This means that I'll sometimes spill some fluid on it or have some glue dripping on it. This is easily wiped off of the oiled surface, but the non-sealed MFT sucks in everything that comes near (a brown ring from a coffee cup for example =).
Booth sealed and non-sealed MFT will do it's work. IMO it's a matter of aesthetics.
I have used hard (extra dense) MDF for many of my surfaces in the shop (home made MFT, SCMS-station, bandsaw extension table, drill press table). The surface prep I've found fits my needs best are the following:
1. Sand to 400 grit
2. Oil with furniture oil (the same I use for benches and oak table in the house)
3. Buff (kind of) in the the oil with a 600 grit pad on the Rotex.
4. Oil again and maybe again.
It's absolutely not necessary to sand all the way to 400. But I like the semi-shiny surface I get from buffing (I don't know if this is called buffing though). If I don't go to 400 this will show. On the other hand you will soon get scratches way bigger than your sanding effort. It's just nice to have a nice surface for a couple of days, I recon =)
At first I used hard wax oil. But I didn't like the plastic-like surface this left on the MDF.
//Michael
