Had a very similar decision to make recently ourselves, what we ended up going for is over on my sort of build thread
Post in thread 'Roadtripper's Caravelle tinkering'
Roadtripper's Caravelle tinkering
Think about what you are going to use it for.
A cassette awning is always there ready to use and takes up minimal interior space (winding handle/tie down kit) but on the downside often needs permanent mounts, raises the height above the magic 2m "anti van" car park barriers and makes you look like a camper (or possibly a plumber/electrician). Most are only really happy in good weather, they can be made more weatherproof but you need to take extra steps like rain strips, extra roof bar to stop puddling, tie downs etc.
You can also just have an awning rail added. That let's you pack a simple sail type awning - and small ones with poles take up minimal space. If you are camping in heavier weather it means you can use a sturdier drive away awning or link to a full tent. But then you do have to transport those things.
If not permanent change is your main reason then your options are basically using your roof rail/bar mounts. Either get one of the simple C rail options or consider the F40 Van top mount cassette awning. Pretty much everything else is going to involve glue or screw, usually both. If you have roof bars then there are some tie on options. There are magnetic/sucker options about but generally not well regarded and if they slide or break free likely damage to the paintwork.
We went c rail and awning as it was the simplest and cheapest place to start. If you go with c rail there is a great thread about on how to mount this slightly clear of the roof and keep the roof water tight. If you go with the F40 Van then likewise some good advice on using PPF to protect your roof from rubbing on the forum - I'd also consider self fitting so the two mounts are sealed to your roof with something reversible like neutral cure RTV silicone rather than something more permanent like Sikaflex that an installer will likely use.
