NGC Morgan Dollar?

Red Book prices the 1884 (assuming plain) at $30.

The 1890 (again assuming plain) is listed at $35.

Even though the 1884 has a lower mintage but are available in AU and up. They could have just stayed at the mint or in banks. Now the 1890 which has a larger mintage seems to be scarcer in higher grades AU being one of them. The reason could be more were melted than is know so the coins mintage really does not mean much. Also the 1890's could have made it out west and were actually used so high grade examples are scarcer. Now a point on the AU grade, the common AU-50 does not have to have mint luster neither does an MS-60. Now a AU-58 coin does have to have some mint luster and often a lot. Most AU-58 coins make MS-60 coins look like junk. To be AU-58 it can not have that many what us old timers call bag marks (nicks and gouges from being in a bag of 1000). An MS-60 coin can have little mint luster and be beat to heck. A wise buy is always an AU-58. Now to answer your question, you actually answered it yourself when the 1890 caught your eye. Buy coins by their eye appeal not their grade or the name on the slab. A nice problem free coin always sells well down the road. The prices are a little high on both of them. That is no sale.

Based on Coin Values (Aug ed) the 1884 is a better value.

Concerning the grade on the 1890. MS means Mint State, which means that the coin shows no sign of being circulated. Mint luster not withstanding, if a coin shows signs of being circulated it can not be classified as MS. It may be a high end AU or what some call a slider but it is not mint state.

Have fun adding to your collection.