I have two horizontal mills Ian – both fitted with vertical heads (but no quill feeds to either).
The larger mill (Victoria H0) tends to be kept in vertical mode – broadly because it is the larger machine but also because the vertical head is very heavy and takes some effort to put on & take off. The smaller mill (Atlas MF) tends to be run mostly in horizontal mode because there are some things where this is more useful for the work I do. I do drill on both mills – on the larger (and heavier) mill, I tend to just 'spot' the holes for later enlargement (on either the drill press or even the lathe) unless I feel it should all be finished in one run or I'm going to bore it. You cannot really 'peck' drill (well you can but it gets very tiring!) using the knee movement.
With the MF – I drill horizontally using a chuck in the MT2 spindle – so depth is put on with the cross-slide, which takes a bit of getting use to but can work very well for some work (e.g. long rows of holes for instance where the Z-axis is fixed). This generally works well enough but my larger MT2 drill chucks (plus drill bit) do not always leave too much room to actually drill and/or change the bit (without moving the table over).
So whilst in my view a horizontal mill (with vertical head) in many ways gives you the best of both worlds generally – I would certainly like a quill arrangement on the larger mill's vertical head which would then be perfect. I've also thought about a making a small vertical 'quill' head for the MF – but it's not very high up on my TUIT list – so I guess I'm managing to get along OK with what I have already. I also have a small Cowells drill and a large Warco 12-speeder for my general drilling needs – so that helps quite a bit in this area…
Not sure this will help you decide what to do but these are my two pennies worth (in old money!) 
Regards,
IanT