I can't comment on which is better as to inside or outside shades by experience but came to the same conclusion as you and went with an outside shade.
I was preferring to not put a bunch of snaps or Velcro or twist locks on my MH and wanted ease of installation and when I was researching I was in Key West with a bunch of RV's, some of which had the magnet shade and they admitted (and I witnessed) that sometimes with a high wind gust (30 or more) that it might get blown off if the wind angle was right, but they did stay on most of the time just fine.
I also saw a "Prompt" brand sunscreen http://www.rv-sunscreen.com/ and it doesn't use snaps, but instead straps to the side window frame, and works great. The people I talked to with this said theirs has never blown off even though some mounts use suction cups depending on the window/door configuration. My MH is a gas unit and I can install it myself, from the ground, in less than a minute without a ladder but that might not be true for a taller DP. You will need to be able to reach about the bottom 12 inches of your side window. You can see how it mounts on their website and I like mine a lot.