I ahve an atlas horizontal mill (smaller than yours) and an Asian round column mill drill with an r8 spindle. I bought the horizontal first and used it for 6 months. The mill drill was an Ebay bargain @ $450, like new, located 2 hours drive away.
I have never used the horizontal since.
Not saying you can't be happy with a horizontal mill, particularly with a bigger more solid one, but.........
well first of all, though I have many good Hz cutters and spacers, and the overarm works well, bearings good, Imainly used the thing with collets, end mills and an angle plate to hold work on, no overarm. That works okay for small stuf, but last summer I resurfaced the head of my Ford tractor on the vert mill drill. If the horizontal was big enough, I could have done the same with H mills on the horizontal, assuming I had enough Z travel to the table to fit a big cutter on the arbor plus the height of the head, but, if not hard to imagine doing it on an angle plate.
H cutters are expensive compared to end mills, and carbide if you need it is going to be rare or pricey by comparison.
Questions to ask is what is the taper in the spindle, Z travel, does it have a good selection of spacers, what kinds of speeds are available......
Up to you on everything of course!
But if I had to give up one mill it would be the H, not the mill/drill..
ps. if space wasn't aan option, and I planned future purchases, AND I was attracted to older machinery, I'd chose a vert mill and a shaper over a vert mill and a H mill. The shaper is truly a different piece of equipment and can do different things well, while a H mill is kind of the same as a V mill with some limitations. I would also say, the larger the H mill the better, and more useful (though more costly to equip). I think size matters particularly there, more than even the others.
Oh yes one more thing -- if you anticipated doing a lot of slotting, particularly multiple slots in a production piece, that might favor the H mill.
