A List is an IEnumerable, via an intermediary - if you browse the Reference Source:
public class List<T> : IList<T>, System.Collections.IList, IReadOnlyList<T>
And a IList is
public interface IList<T> : ICollection<T>
And:
public interface ICollection<T> : IEnumerable<T>
So in effect they are the same when it comes to actually using them, and speed difference will depend on exactly what you are doing.
In the case of your code, the first one will be significantly faster, because it doesn;t execute the Selects - it just sets it up for later execution. Unlike your List code which does execute the selects because the use of ToList "collapses the operation" and actually requires that the collection is iterated.
If you want to know what is fastest, then test it!
There is a Stopwatch class in System.Diagnostics for just that purpose.