Even the latest builds have the options of at first, at last, at center, at cursor, collapse, as all the previous versions before, but the builds I use are from graphicall.org so the options go like this:
- at first
- at last
- at center
- at cursor
The first two options work on selection order meaning two selected verticies will either be welded at the first one selected or the last one selected. Center will be at a point between two or more verticies, at cursor will be at the 3D cursor and collapse will collapse the object.
You will only get three options in the Alt-M option if you have an object that you go straight into edit mode with and have all verticies selected, or select all verticies in an object or select verticies in a way that does not respect a selection order such as lasso select and box selecting. You will only be presented with the first and last options if you select an individual vertex first or all verticies individually.