I couldn't agree more with troopersam, if you can't pay for it in cash then you should wait until you can. As much as I love vacuums, I think it would be a bad idea to go into debt for one. Especially since you have several already, and many other good machines can be purchased for a fraction of the price. There are a million other things you will be in debt for in life, school loans, cars, a mortgage, dont make a vacuum one of them.
As for the new Guardian, I would wait for a used one to pop up on eBay. As much as I love Electrolux, from what I've heard, the Guardian Ultra retails for about $2,000. While some others may think the price is worth it, I personally can't see paying that much money for a machine that has the same body that has been around for nearly 25 years and merely has a HEPA filter fitted into its top cover. I love my Electrolux Renaissance and if the new Guardian Ultra was the same "R" style body, then maybe it would be worth it, but its the same body as a HiTech-2100.
Miele makes some wonderful canisters which clean and filter exceptionally well, especially when paired with the SEB236 power nozzle. The cost of a TOL Miele with an SEB236 is about $1200, and you can usually negotiate the price with most vac shops. I've owned 3 different Mieles canisters and I find them superior in many ways to Electrolux tanks. They have tremendous suction, a sealed filtration system, HyClean bags, quiet operation, and a superior HEPA filter. The latest SEBO D4 also looks like it might give the Miele a run for its money, as it has many similar features, a <span style="text-decoration: underline;">28 foot cord</span>, and its less expensive than the TOL Miele. I have never used a Rainbow, but they certainly seem like interesting vacs. And its seems like they are fairly reasonable to rebuild, along with having a very low operating cost, which may outweigh all the additional benefits of both the Miele and Electrolux, plus you already own the machine and it only requires a minimal investment. Sorry to rant but when I purchase something, I like to consider all the different factors of each of my possible options.