I have an bench-top Atlas horizontal mill without any tooling. It only
has a 7/8" arbor and there are very few 7/8 cutters (on eBay). I see 2
alternatives: buy (?) or make a 1" arbor and buy cutters for it, or buy
end-mill holder(s) and end mills.

I'm leaning toward the end-mill approach for these reasons:
- the 1" arbor is going to be expensive to buy or over my ability to
make
- you can do more things with end mills (e.g., dove tails and slots)
- there are more end mills available than arbor cutters
- end mill cutters are easier to change than horizontal arbor cutters

The only thing that I can think of that I'd be giving up is the greater
rigidity of the horizontal arbor.

Am I missing anything?

Thanks,
Bob