I'm coming late to this topic but thought I'd post a few words about the Amolco as I have one (with the Myford ML10 base fitting to suit my lathe).
The Rodney and Amolco each have their strengths and weaknesses:
Work Envelope — The spindle-to-column base dimension is similar: 4" Amolco, 4-3/8" Rodney. The maximum headroom above the cross-slide surface is: Amolco 8.5", Rodney 6". The cross travel is, of course, dependent on how much travel your cross-slide has — my ML10 with 'long' cross-slide has about 6-1/2". So, the practical horizontal envelope is about 6" x 4" with the balance in favour of the Amolco on headroom.
Spindle Speeds — Rodney: 320-2750, Amolco: 325-1600. Similar speed range except that the Rodney has the advantage at high revs, useful for very small milling cutters. At the low end the Amolco is too fast for operations like gear cutting and slit-sawing in iron/steel. Actually, this was where I thought the Rodney should have the advantage i.e. if it's driven by the lathe spindle, surely it has an even lower speed range employing the lathe's backgear? If not, then the speed ranges are comparable in the low and medium ranges.
Controls — On the Amolco the only control you have is a single vertical leadscrew with graduated dial handwheel to move the head and set cutter depth (not ideal for drilling). On the Rodney you also have a lever quill. With only 6" of headroom (minus chuck, vise and workpiece depth), though, I'm not sure how practical drilling would be on this machine.
Autofeed — As regards the Rodney, there is none. Because it has independent motor drive, the Amolco can use the lathe's leadscrew to provide autofeed across the short axis (4") of the workpiece. This is a definite plus if you've a lot of material to remove via multiple passes.
Quality — Well, the Rodney certainly had the Myford gloss and a premium price to suit! While the Amolco was cheaper, it was 'of good quality construction' to quote Lathes UK. I got good service out of it prior to getting a standalone mill (and may yet again if I have to downsize!).
I agree with the consensus — do all you can to shoehorn a mill into your workshop if you can. But if that's not possible, don't pay silly money for one of these attachments. I wouldn't pay more than £250 tops for either kind.
Joe