I don't use a contact wheel that often but when I need to hog off a lot of material fast, as in profiling some metal before welding or reducing the size of something with precision, then the contact wheel is hard to beat. It does grind a slight concave radius in the part, however, so if this is a deal breaker then it might not work for you. One thing for sure; if you use it for your drive wheel on your machine you will have less slippage.
The platen allows for fine control and produces a flat, precise surface. With a good tool rest it is very hard to find a better tool for grinding HSS lathe tools (that most of us can afford). Good for you that you're going to use a Pyroceram liner. It is one of the most valuable mods you can make to a belt sander in my opinion.
Matt, since you're building your grinder from scratch I would encourage you to come up with a really good tool rest that you can repeatedly set to precise angles. I know you use an Atlas and you need lathe tools that cut with less cutting force to get the best performance from the lathe. The keys to that lie in the tool rest and a platen that will stay flat.