The answer really depends on what kind of woodworking you want to do in the future, and how much you intend to spend on woodworking tools in the near future. If you're buying power tools only for the workbench you need to build now, the circular saw is probably the cheapest tool that will get you nice results. However, if you want to build more complex projects such as furniture, like another poster said, the contractor table saw is by far the most used woodworking tool in my garage. It's often the fastest and most accurate tool for most projects except for cutting down large plywood sheets, flattening or straightening lumber, or cutting curves or trims. But the cost is high enough ($500+) that I would not recommend getting a contractor table saw until you're sure you like woodworking. And many people don't recommend getting a cheap table saw since it may nullify the main accuracy benefit of getting one in the first place.
If you do choose to build your workbench with a circular saw (a good choice for sure), I would not recommend buying something too expensive. Since anything that needs accuracy gets cut on the table saw, I only use my circular saw to rough cut plywood sheets, which then get cut to specific dimensions on the table saw. For this use, I have a $13 (refurbished) Ryobi corded circular saw that gets the job done perfectly well. The only reason I can think of to buy a premium circular saw is if you are a contractor on jobsites and need the durability. For many woodworkers in a shop, the circular saw is not regarded as a tool for accuracy, so I don't see the value in paying a premium. It will still do a great job on your workbench project (which isn't fine furniture anyway).
If you think you will do a lot of home remodeling projects in the future (installing crown molding or floors, for example), a miter saw is really useful for cuts of long lumber, especially angled cuts. It would be an upgrade over the circular saw for your workbench project in speed and accuracy. Personally, I build furniture for my woodworking and don't do any home remodeling, so I haven't had a need for a miter saw and don't plan to get one. If you ever plan to get a table saw, it can do everything a miter saw can do and with greater accuracy, so you might not need both. But if you have the need, this would be a good time to get a miter saw which would make your workbench project far easier and faster than the circular saw. Hitachi makes a good, cheap and reasonably accurate miter saw for around $100, sometimes less on sale. Good luck!
Edit: I looked at the workbench plans you linked in a later post. Since you will be cutting down a large plywood sheet for the table top, the best tool choice is going to be a circular saw. Or a miter saw/table saw for the frame and a circular saw for the top, if you have the budget.
