JFrank - First of all, let me congratulate you on the purchase of your Thermax AF1. If you're happy with it & how well it cleans, great.
However, regarding what Devin, Blackheart, said in his reply to you, I totally agree with him. The Thermax vacuums have always looked bulky & poorly designed to me. And let's not forget that Carl Parise, the founder of Thermax, started out designing & manufacturing carpet extractors, NOT water filtration vacuums. It was merely a afterthought, & looking at early models vs their extractor designs, I would bet they heavily based their designs on their existing extractors & made modifications to make it into a workable water filtration vacuum. To me, they look like a design that would perform poorly, & I have also only heard bad things about them, not good.
The other problems you are going to encounter is maintenance & getting repair parts. Since the Thermax does not have a separator & relies on filters, you will find it's a LOT more hassle than the Rainbow to use & clean up after on a daily basis. Also, Thermax changed ownership a few years back, & they released a newer model, the AF2, that looks cheaply built, & there hasn't been much news coming out from the company. I also don't personally know where there are any existing DTD distributors for the company, & that would prove problematic in getting repairs done. Based on the lack of recent news or any information from the company, I would bet they are one of the many DTD brands that is either just hanging on by a thread or they have already gone out of business.
For those reasons, I would 100% hold onto your Rainbow E Series.
Now, as for the complaints about your Rainbow that made you buy the Thermax. If you want a longer hose for the Rainbow, they do make optional 14 foot & 30 foot hoses available. I have linked to them below. As for the power cord being too short, you could take the Rainbow into a vac shop & have them fit on a much longer cord.
As for your observation that the Thermax seems to be picking up more dirt than the Rainbow does, the reason for that has to do with the power nozzle design. Rainbow redesigned their power nozzle for the D4 Performance Edition model, so that the belt is in the middle instead of at the sides. Unfortunately, due to the way the air path is designed, it also chokes off some of the vacuum's airflow. That's the reason why your Thermax is picking up more dirt - it uses a conventional power nozzle design with the belt off to 1 side, instead of in the middle, so the air path is a conventional design & does not choke off the vacuum's airflow. I would suggest trying the power nozzle I linked to below, that will greatly improve the Rainbow's performance on carpet. Both the Thermax & the Rainbow should produce about the same amount of suction & airflow, so that shouldn't be a factor as for why the Thermax cleans carpet better. It is made by Wessel Werk, a German company that supplies power nozzles, floor brushes & rug/floor nozzles for the central vac industry & for many OEM vacuum manufacturers like Miele. A quality design that will last a long time, with the added benefit of a geared belt that should last the lifetime of the power nozzle.
Your Rainbow is a really good vacuum, if you prefer water filtration vacuums it is one of the better makes. I would strongly suggest you hold onto it & make the improvements I pointed out so you will be more satisfied with it. You may find that you like both vacuums equally as much, you won't have wasted your money buying the Rainbow, & you will already have a backup vacuum for when the Thermax does die & it cannot be repaired.
Best of luck
Rob
http://www.goodvac.com/Rainbow_e_series_vacuum_electric_hose_parts_s/805.htm
http://www.goodvac.com/E_E2_Power_Nozzle_by_GoodVac_p/gv-nzl300e.htm