It depends on what you are going to use the XXL for depends on what you need. On the Carbide3d site under the HDZ they very clearly say the HDZ is more than most need. That said I will not tell you how to spend your money but if you have no experience with a CNC Router start with the stock setup and build up to the point that you need to upgrade. Most never need to modify their Shapeoko to get great results. But again I dont want to tell you how to spend your money but here is the advise I have given before:
The CNC Router is only about the first third of a budget for this hobby.
The router bits, calipers, sanders, finishing supplies, wood etc are the second third of your budget.
Software can be another one third of your overall budget. You can get a free Fusion 360 startup license, you can buy various Vetric levels up to $1500 down to $150. There are others both pay and free but you get what you pay for.
Aside from the financial aspect of CNC Routing is a commitment to the time required to learn the physical part of the CNC Router, the steep learning curve of the software, the time to watch learning material on the internet and the finally the perseverance to go through the process and produce useful and/or functional objects.
So my advise is to start with the basics, master that and then move on to bigger and better things like the HDZ, spindles and other upgrades like steel belted belts, linear bearing rails and so forth and so on.
I just recommend you start with baby steps, learn how to crawl, then walk, then run, then chew bubble gum and walk at the same time. If you choose to run first it leads to disappointment as many have found when they did not dedicate the time and energy to learn the craft. Many CNC Routers are sitting in garages and basements gathering dust and the “Why did you spend so much money on that thing” from your significant other.