If security is your concern, go for a hard cover.
There are roughly three types:
1. hinged.
2. Folding
3. Rolling
I think color-matched hinged type looks the best.
I think functionally, 2,3 will be better since they don't require taking the cover off to carry larger items (most of time, anyway).
I think for colder weather, folding type would be better. I got BAKFlip F1 recently for all the reasons mentioned above. It could also withstand up to 500 lbs on top of it. I like it, but the other day, when we had wet snow and freezing weather, I couldn't open the cover. It was frozen solid. I did expect that it would be more suitable for colder weather. But then again, the rolling type would perhaps be worse under such situations.
Now I am thinking that the hinged type might be better for colder climate. Perhaps someone with hinged type could provide some input.