There are basically three choices of collets for the Atlas 6", 2MT, the Atlas M6-751, and the smaller ER's.
If all that you need to do is hold milling cutters, the 2MT is probably the least expensive option. AFAIK the 2MT collets only came in 1/16" increments and each size only has a gripping range of about +/- 0.005". For gripping parts, you would be limited to 1/16" increments. So not a good choice for general use.
The M6-751 sold by Atlas is rare in a complete set and is almost certain to be the most expensive option. Plus the largest diameter that is available is 9/32". So I would judge it not a good choice unless your only need for collets was working on parts always under 5/16" diameter and you find the set cheap.
ER32 or ER25 holders or closers are available from several sources, or buy the nut and make the rest. There are three types or styles.
The first and least desirable is mounted on a 2MT arbor either with or without a draw bar. Avoid the one without a drawbar as sooner or later, the arbor will get loose in the taper and probably ruin the part. The arbor with drawbar works OK except that you can't pass long parts or stock through the spindle. And you need to use a spindle thread protector with it. However, it and the 2MT's are the only ones safe for machining with in Reverse.
The second type is mounted on a face plate that screws onto the spindle. The only disadvantage to this one is that it sticks out farther than the others. It is advantageous to a seller as the same chuck or closer can fit a face plate for any lathe.
The third type screws directly onto the spindle and in most cases is the preferred type.