DesignSpark is quite severely limited so I always advise against it. In particular, it doesn't allow (without paying extra) the ability to export STEP files. STEP files are representations of your model that can be loaded into another CAD application and relatively easily edited. You don't get the full history so it's not as easy as editing in the original application, but it's a lot better than nothing.
Let's imagine you're using Fusion 360. If you get in the habit of exporting STEP files regularly (
which you should), then if Autodesk decide to further limit the free version of Fusion 360, you'll still be able to load your models into another CAD application to tweak them. If you haven't exported any STEP files, you've lost your model. If DesignSpark limit functionality in some way, you've lost your models and there's nothing you can do about it apart from paying the makers of DesignSpark lots of money.
I would strongly recommend you have a look at the free version of Onshape:
https://www.onshape.com/en/products/free
Onshape is completely free and (in my opinion) vastly superior to Fusion 360 - in particular it is a heck of a lot more intuitive and similar in interface to other CAD tools like Solidworks, FreeCAD etc. Fusion 360 seems to have deviated from a few of the norms and I've found that while I can switch between Onshape, Solidworks, Solid Edge, ZW3D, NX & FreeCAD quite easily, Fusion 360 always needs a bit more thought. If you get used to Fusion, you'll find it harder to switch to another CAD package than if you start with something like Onshape.
Onshape runs in a browser. That has some advantages and disadvantages. Advantages are that (1) it runs on pretty much any computer and (2) some of the modelling happens "in the cloud" and hence your computer doesn't have to work as hard (when I use Fusion I find my computer fan is going crazy most of the time trying to cool the processor down). The main disadvantage is that the models are saved in the cloud so there's a risk that, if they remove the free version at some point in the future, you'll lose access (something that is easily mitigated by regularly exporting STEP files). The other disadvantage of the free version is that your models will be publicly available (not that anyone will ever find them in amongst the millions already there), so it's not ideal if you want to commercialise your model.
Another good option (again vastly superior to Fusion in my opinion) is the
Solid Edge Community Edition, which is also free (for non-commercial use). The main disadvantage of if (compared to Onshape or Fusion) is that there aren't quite so many tutorials available. Having said that, the tutorials there are seem to be very good quality (vs Fusion, where a lot that you find are for very old versions and no longer applicable). Solid Edge has two different modelling "modes". One (called "Ordered") is much like other CAD applications; the other (called "Synchronous") some people (not me) find more intuitive as it's closer in style to things like Sketchup etc.
FreeCAD is good, but the user interface (especially the different workbenches) can take a little bit of getting used to. It's also not quite as polished as the other ones, but it's getting better every release and, as it's open source, you can be sure that you'll never lose access to it.
Probably the best low-cost (rather than free) option is (in my opinion again) the
"maker's" edition of Solidworks (the one on the right-hand side of that link, not the cloud version). About £35 a year ($48) for a seriously powerful and really quite intuitive professional level CAD application.
The only reason I would pick Fusion over the others is if I wanted a free CAD application that I could use for commercial purposes (and even then I'd have another good look at FreeCAD first).
Incidentally, PrusaSlicer (which takes your model and slices it for 3D printing) can read STEP files as well as STLs. I can't remember the last time I bothered to export an STL file (which are bigger and not very easily editable compared to STEP files): I just export STEP files and load them into PrusaSlicer. This has the double benefit of giving me a model that can be printed and also giving me an exported STEP file in case I ever have problems with the CAD application I've used in the future.
). Like all programmes it has its good bits and those you would like to see improved. I tried Fusion 360 but couldn't get on with it. 
