Experts recognize two major varieties on 1828 Dimes: Large Date and Small Date. The Large Date is a holdover from previous years, while the Small Date establishes the pattern for later years. The difference between the two is obvious in a side-by-side comparison. The Large Date can be called a Curl Base 2 and the Small Date a Square Base 2, though the difference between them is not so marked (especially on worn coins). In addition, the stars are large on the Large Date and small on the Small Date. The Large Date is comprised of a single variety (JR-2). Likewise, the Small Date is represented by only a single variety (JR-1).
According to the PCGS Population Report (as of August 2011), the Large Date 1828 Dime is scarcer than the Small Date and only 5 Mint State examples of the former have been certified by PCGS (the best of which is a single MS-65).
Sources and/or recommended reading:
Davis, David J et al. Early United States Dimes 1796-1837. Ypsilanti, MI: John Reich Collectors Society, 1984.