The Joie is more compact front to back, but the Dualfix Plus has the benefit of a higher weight limit as said. And the Britax seats in general are nicer to use than the Joie ones, IMO.
Can you go to a stockist and try both out? That would usually be the best thing to do, it's a good idea as you'll be able to see how the seat fits in your car in terms of angles and front seat space, what it's like to spin, try it out with your child to see if they seem to look or act more comfortable in one or the other.
I know you can save a little bit of money by buying online, but it generally is really beneficial to have that experience of being able to see the seats in the flesh, try out the recline/spin/harness/height adjustment to see how they feel. If you can find a local independent or specialist retailer, even better as they tend to be a bit more passionate about baby products than some Saturday kid at Halford's, so you typically get better advice, and can usually go back for after sales support e.g. checking that your child is still fitted correctly now that they are older, advice on what to do when an older child gets their arms out or if they fall asleep and their head falls forward, if you're ever unsure about whether to take out an insert, whether or not to change to forward facing, or if you have any problems with the seat and need to send it off for repair, it can be worth the extra £20 or so for all of that service and not having to deal with faceless online bots.
To find an independent retailer, go on the Britax or Joie website and look under "Stockists", then put in your location and look for any that come up which are not chains.
And yes, if you come across an independent ERF specialist, you might also want to look at the Swedish type seats, these are even more compact than the spinning seats and they last longer - up to 125cm rather than the 105cm that spin seats last, which is a good couple of years extra.