It adds more cleaning, since you are experimenting with fleece, why not start there and then see how you do with the extra laundry, if you can handle it then sure why not add a loft! Price wise though, the more you add the more materials you need. Unless your plan is to put in a disposable kitchen, that to me is the best thing to add first.
Its not hard to clean but it is more to clean, all of my upper level stuff swings up and out of my cage so I can lift out my kitchen and my patios when I change my cage. No matter what you do it takes more time but it's not harder. Other options would be to put the loft against the back and use a plastic tub for the loft which would be 100% waterproof (better than coroplast and easier to clean in my opinion. You can get very large ones at walmart. If you get a clear one it will let more light in.
My stuff is all patio height (half grid height) so that does make it darker under the loft, Loft height (adding a whole second upper level with grids) would bring in more light. Patios are much cheaper though because you use a lot less material to build them. If your goal is to make a patio kitchen then I think it's worth it. It also frees up space on the main floor and provides overhead security. If you put the kitchen under the loft or patio it hides the mess and encourages them to use it more. Really its up to you and anything you do you can add to or take away later if it doesn't work for you. All 3 of my girls took to my upper level as soon as i put it in. It's a very low incline so that might be why.
The only problem that I've had with my loft/kitchen is the loose bedding coming down the ramp onto my fleece. I have put a little piece of coro in the opening but some bedding still escapes. But if you used fleece in your loft you wouldn't have to deal with that I would suggest getting a loft though, it gives your pigs some extra exercise and a place to get away from each other!
I love love love my loft! It keeps my cage much cleaner. My girls pooh and pee 90% of the time up there. It also keeps the hay in one place. I have carefresh in my loft so you can pick up any loose bedding in the fleece and put it back. One thing I don't put up there is the waterbottle because it make it too crowded (between the food bowl, hay, and veggie bowl.) It also makes spot cleaning so easy! Just scoop the bad bedding and put more in. When its time for a full clean cage dump it into a garbage bag, wipe, and refill with bedding! Super easy! I have love my loft! Also when they are upset at each other one goes to the loft and the other stays on the first floor until things cool off. They also have to go up a ramp to eat so it creates a little more exercise. You should consider a loft.
Another vote for no loft from me... The loft I had made the cage a real pain to clean, and my guinea pigs seemed not to enjoy it as much as I'd hoped. They seemed annoyed that they had to go up a ramp to get their food haha. And especially as guinea pigs get older, a loft will put extra stress on their joints. Your cage size provides an awesome amount of floor space already for your pigs. If you don't end up building the loft, you can just go crazy with fun cage accessories! You could also add a separate coroplast box kitchen area (2x1 or so) to contain all the hay, pellets and water from off the fleece.
I think it's a matter of personal preference and guinea pig preference. I had a loft in the first C&C cage I made but eventually took it out. My guinea pigs tended to congregate under the loft and ramp. One claimed the loft and would block the entrance so none of the others could go up there. I finally took the loft out and expanded the cage and found that they ran around a lot more. If I were you, I'd try your cage with a loft and without and see what works better for you.
I like my loft and my gals like it too. I keep their hay and pellets up there and they have no problems going up there as often as they like. I'm a tall guy and it also makes it easier for me to interact with them if they are upstairs. I can comfortably pet them or hand-feed them treats without bending over. They've started to know when I am bringing a treat and they will run upstairs to greet me.
My loft is full height so cleaning under it is no problem. I use the area under the loft for fleece forests or other things that I can suspend from above.
My only real frustration is that they sometimes like to pee and poop with their butts at the top of the ramp which causes it to trickle down and make the bottom of the ramp carpet damp and gross. Since I got a second carpet and swap them out and wash them along with the fleece regularly it is less of an issue.
Ok thanks everybody. I am trying to switch to fleece so I am going to keep my single level for now and maybe add a loft a little later. And my boys just had a fight last night so adding a loft right now might not be a good idea