I can only tell you my thoughts about them, other people might have totally different views.
Even though a vertical slide is cheaper, and a lot of fantastic pieces have been made using them, unless you are rather skilled in that use, you might find yourself foundering when coming to machine even some basic shapes.
At one time, vertical slides were the ONLY way of doing milling, as mills were just too expensive for the average shop. But things have changed a lot since those days, and if you are lucky enough, you can come across some good second hand mills that come with the basic tooling included, where someone is going to a larger size, or just giving up because machining wasn't for them. If it was me buying my first mill, that would be, and was the way I went.
If buying from new, tooling to use on the mill will come to approximately 30% to 50% of the mill cost extra on top, and that needs to be factored into the overall costs involved.
Here in the UK, lots of people used vertical slides when they first started out, but now they are languishing, gathering dust, under thousands of workbenches (the vertical slides, not the people). Rof}
I gave mine away many years ago to someone who was just starting out, but he only ever managed to do very basic stuff with it, and eventually bought himself a small mill, second hand from someone I put him in contact with. I suppose that slide is now sitting under someone else's bench by now.
John