We use Dixon at school and I think it's great.
I asked my Economics teacher whether or not I should get additional textbooks, especially Riley since I got his Year 11 text book from my cousin and I found it quite useful. I say 'quite' because although Riley goes into a lot of detail (which is great if you struggle on getting the analysis bit in essays right) he can be quite difficult to understand if you don't know the basic/foundation, which Dixon is amazing at covering.
Here's the interesting bit: my Economics laughed when I mentioned Riley's name as it turns out that they attended university together and uh, he's not a big fan of Riley AHAHA. Said something among the lines of 'always pushed me to the edge' etc. Thought this would be interesting to share LOL
Regardless, he says if you use the two text books together it's a safe option but it isn't necessary.
Both Dixon and Riley are great, it depends on where you lack personally as to decide which one to use.
In short, I think Dixon is great if you need the basics, get your head around the theories etc.
Riley is when you want the in depth explanations which doesn't work well for everyone.
If you successfully use both well together and research current economic data yourself I think you are pretty much covered.