What's the difference between the United MileagePlus Select card and the Explorer?

The biggest difference between United MileagePlus Select and Explorer is that the Select card is no longer available to new applicants. 

The United MileagePlus Select card allowed you to earn 3 miles per dollar spent on United tickets, 2 miles per dollar spent at restaurants, grocery stores and home improvement stores, 2 miles per dollar on tickets purchased from other Star Alliance carriers and 1 mile per dollar on everything else.

In contrast, the United MileagePlus Explorer offers 50,000 miles for spending $3,000 in the first 3 months. In addition to those, you get 2 United Airlines miles per dollar you spend on purchases from United Airlines and 2 United Airlines miles per dollar spent at restaurants and hotels when you book directly and 1 mile per $1 spent on all other purchases.

Other benefits worth mentioning are free checked bag and priority boarding, two United Club passes annually and primary rental car coverage.

In essence, the Select's earning ratios for United purchases were more lucrative than what the Explorer offers. That also applies to grocery purchases and home improvement purchases, where the Select was twice as rewarding as the Explorer.

However, the Explorer has the edge over the discontinued Select when it comes to hotel reservations booked directly through the United Hotels portal, with 2 miles per $1 spent versus 1 mile per $1 spent.