May I suggest that you use oil paint anytime you paint a floor. Properly cured alkyd (oil) paint has the wear resistence to stand up to foot traffic and abrasion which a latex product absolutely will not have. Jay's suggestion to prime the floor prior to painting is a good idea and you should again use an oil-based primer. There are special formulations by most major brands specifically for painting porch floors and use the term "Porch and Floor" in their labeling. Just make sure it cures properly to allow the finished coat to get the hardness it's designed to have. As for tiling later, you mentioned you have a plywood floor, which I will assume is standard 3/4 inch plywood. Whether you paint or not, you will need to add another 1/2 inch of substrate to insure a flex free base for the tile. Many a failed tile job is the result of too thin a substrate. May I recommend installing a 1/2 inch concrete tile backer screwed over the plywood before tiling and you will have a beautiful, long lasting, trouble-free floor. Have Fun !!!