“Pastor” is a spiritual gift while “elder” is an official role of leadership in the church. Ephesians 4:7 articulates that gifts are in view; all the entities named in v. 11 are spiritual gifts. Those with such gifts should lead, but not all leaders have such gifts. Elders are required to be able to teach, according to both 1 Timothy 3 and Titus 1; hence, their role is also that of a leader, though they may not have the GIFT of teaching (just as Timothy was asked to do the work of an evangelist, even though he may have lacked that gift). In other words, it’s not an either/or, it is a both/and situation.
Today, a pastor is one who has the office of pastor; while in the early church, this was more considered a gift. 1 Peter 5:1-2 is a key text, for it declares that elders should pastor the flock of God. (The word in verse 2 normally translated ‘shepherd’ is the same verb for ‘pastor’ used elsewhere; when used in a spiritual sense, as opposed to a literal shepherding of sheep, it always refers to a role of leadership such as pastor, as it clearly must here for the phrase, “the flock of God” makes that plain).
I personally see a danger in having only theologically-trained pastors running the church. Lay elders need to be heard as well. All must bow to the authority of Scripture, but if there is a single pastor at the top who dictates to others what the Scriptures mean, then he functions as a sort of Protestant pope. This in not a healthy situation.