Nice sled, but of MDF, I bet it's heavy. Having the tracks will allow you to really secure your stock to the sled, which eliminates the error you can easily get from movement during the cut. You could also consider some PSA sandpaper to give some grit. 

I have a pile of sleds, all for different reasons, but sliding something through the blade well supported is a great answer for movement which can queer any cut. So for me, and my adult tremors I love them. 

On 3 of mine I have a deck for the offcuts to slide out onto. All 3 of them are locked in place, and don't move, much like using an outfeed table, both safety, and protection for the stock you are spending time trying to get a very nice cut, no bouncy slidy out of control where the offcut can fly up onto the blade and become a rocket headed at your head. On my decks all 3 of them are from the same material that I made the deck of the sled from. Offcuts just slide off, and usually just sit there, so my vote would be same height, and locked down so it isn't moving, and possibly then competing with the sled itself. 

If you do a lot of mitered corners on boxes and such, you might also want to consider a 45 degree sled. I use mine for short exact miters, and not so long bevels. Sandpaper on the fence, and deck right near the blade make it hold stock so I seldom have to use the clamps I laid in via T Track. I looked at a lot of designs, but ended up going kinda non conventional. At least it seems, most miter sleds have some kind of wedge shape, which I had a long time ago, and had a lot of issues with creep. Made for some goofy cuts. With this style I find the cuts to consistently be right where I want them. Marie offers a few challenges to overcome, so  I am popping her video on. Plus some of this type have the blade right at the edge. The way she and I did it, leaves you an offcut table that is attached, so it doesn't try to compete with the sled, because it is the sled.