Portland cement comes in a variety of different types. In the United States, these types are classified as Type I, II, III, IV and V. Only Types I and III are necessary for consideration by concrete countertop fabricators; the benefits of Type II cement are generally irrelevant to the concrete countertop industry.
Type I is ordinary Portland cement, and it is available in white or gray. Most concrete countertop professionals use white cement, despite its higher cost, due to its more precise color consistency.
Type II is a moderate sulfate resistant cement, important when concrete is cast against soil that has moderate sulfate levels, but irrelevant to concrete countertops.
Type III is a high early strength cement. It is ground finer and reacts faster than Type I, so the early compressive strength gains are greater.
However, the ultimate strength is not higher than Type I. Concrete made with Type III will have only slightly higher 28 day strengths than concrete made with Type I, all else being equal. However, concrete countertops are installed within a few days of casting, and there is no need to wait 28 days to cure.
Type III is available in white or gray, but white Type III is difficult to find in small (less than pallet) quantities; it often has to be special ordered. Given this, and the fact that there is no need to increase the early strength of the concrete, it is best to stick with Type I cement.
Type IV and V are often used in special construction applications where high sulfate resistance is required or a low heat of hydration is important. Neither of these types are practical choices for countertops.
You will sometimes see cement labeled as multiple types, such as I/II or II/V.
Conclusion:
The high compressive strength of well-designed concrete countertop mixes is determined mainly by good concreting practices, such as low water-cement ratio, not by the type of cement.
It is best to stick with Type I cement and to understand all aspects of good concrete countertop mix design, such as the importance of water-cement ratio.