Will, since you are a pet owner, I would choose the Sebo Felix over the Miele C3. I just recently bought a Felix, and they are great vacuums.
One nice thing about the Sebo Felix is you can get a activated charcoal pre-motor filter for odors, but unlike Miele you do NOT have to compromise on filtration just to eliminate pet odors. The S Class filtration system Sebo uses consists of the HEPA Cloth bag and the cloth cover surrounding the body of the vacuum. With Miele, you have to choose between the standard AirClean filter included in a box of bags, the Active AirClean filter, and the HEPA AirClean filter. The Active AirClean filter is the charcoal layer and the cloth AirClean filter enclosed in a plastic frame, so you don't get quite the same as HEPA filtration. Yes, there are generic HEPA filters available for Miele canisters, but note that if you use them, you invalidate the manufacturer's warranty by using non-OEM parts. There are also generic HEPA Cloth activated charcoal bags for Miele canisters if you wanted to use them.
Since you already have a Rainbow for a canister vacuum to clean under low places and for attachment use, the Felix would be ideal to have a upright for those times you want to grab a upright to use, like for quick cleanups when company is coming over. You should also look at Kirby and Oreck, as well as reconsider Hoover Windtunnel bagged uprights as Brian suggested. All 3 of these brands have the option to use HEPA Cloth activated charcoal bags, and there are also generic versions of the HEPA Cloth bags and HEPA Cloth activated charcoal bags as well. Since you are a pet owner, and would be filling and going through bags more frequently, the availability of generic bags and bag and filter costs is definitely a factor to consider. That's actually one disadvantage of the Sebo Felix, is there are generic HEPA Cloth bags available for it, but they are harder to find than some other models.
Also, bear in mind that if you choose a vacuum that uses normal HEPA Cloth bags and no charcoal filter is available for it, what you can do to eliminate odors is to buy activated carbon in granular form at the pet store to make fish tank filters. When the vacuum starts to get smelly, take 2 or 3 tablespoons, put it on a flat surface and suck it up with the hose. That will quickly eliminate the bag smell.
If for some reason you prefer to buy another canister over a upright, the Aerus Guardian Platinum incorporates a charcoal filter layer on the pre-motor HEPA filter, and the Riccar Prima canisters also come with a charcoal filter as standard on the higher end models and available as a option to buy for the lower end models. However, both these vacuums would have the disadvantage of generic bags being harder to find. Also, just so you know, if you want a vintage upright to collect and use, the Hoover Convertible and Hoover Elite uprights take the same Type A and Type Y HEPA Cloth activated charcoal bags.
Rob