TL;DR: From purely a range consideration the 90 degree mounting is the best because of the orientations your quad might see. You'll want to get the antennas as far away from the frame as you can so that the frame can never block at least on antenna from seeing your transmitter.
When you are considering ways to mount your antennas on a miniquad however there isn't one best way to mount them. Range, quad orientation and durability all play a part in what will work best for you. That said there are two things you should know to understand about antenna placement and why the 90 setup is recommended for range.
- The reason for two antennas.
The XM+ antennas both have the same function. The receiver simply compares their signal strength and picks which one is best. This lets you place them in different orientations so that depending on your orientation on antenna or the other will have adequate signal.
- The radiation pattern.
Generally the monopole antenna has it's best signal reception at a direction that's perpendicular to the antenna itself. Conversely it suffers from pretty bad reception in the top and bottom directions. This image shows the typical radiation pattern. Imagine the wire antenna standing straight up inside this donut. The red directions have good reception and the green are where it is bad.
Now, since you have two antennas you can see how rotating the second one by 90 degrees would cover the deadspots of the first antenna.
These two orientations make for good coverage in almost any orientation. However, it's not perfect, those donuts are pretty flat. Mounting both antennas in a more upright 'V' orientation could provide a lot more even coverage, but only if you don't plan on doing something like diving straight down.
With all that said, realize that those nice donut patterns can be blocked by your frame if they are placed in such a way that the frame can come inbetween you and the antenna. You can see how this would happen in Paul's example. Not to say that's a bad way to do it. I do it that way often. For typical park rages it still gives you that 90 degree orientation and will protect your antennas from prop strikes and crashes more effectively. For extended ranges though, one antenna sticking out the top and one out the back will give you the best range.