One thing to keep in mind is flexibility. As you work at setting fixtures into the landscape and seeing the effects at night, you might decide you need a different beam spread or more or less luminance. You might also see that changes are required as the landscape or seasons change.
In the MR-16 days, you could carry a box of lamps with various beam spreads and wattages. I've been on many jobs where the designer changes his mind and swaps out lamps after seeing the effects.
Now, with the advent of fixed beam spread and fixed luminance LED fixtures (like the ones Jim describes), you are stuck with what you buy prior to installation. If you want to change anything, you're out of luck. This is not a small matter and seriously limits your flexibility in lighting design.
The alternative is to select LED fixtures with adjustable beam spread (by changing lenses) and adjustable brightness by switchable luminance or dimming. As an example, the CAST LED Bullet
has two brightness settings and 3 lenses - that's the equivalent of 6 different fixed LED fixtures. CAST's next generation bullet will have 5 brightness settings and 5 lenses (one of them asymmetric) - that's the equivalent of 25 fixed LED fixtures.
By using adjustable LED fixtures you have tremendous freedom to fine-tune your designs on the fly and accommodate the changing landscape.