Hi Simone, I am new here but I have had alpacas since 2007. I do not have sheep (yet) so I can't give you first-hand experience comparing the two exactly, but here are some of my experiences.
When I first started researching alpacas, I can tell you that all resources I checked listed a stocking rate similar to goats and sheep. As others mentioned, this will vary at different times of the year but my local ag office stated that the stocking rate for sheep in my location was 10 sheep per acre.
My alpacas tend to be just a bit picky on their pasture conditions. They do browse somewhat, but seem to prefer younger and more tender grasses above all else. Sometimes life gets ahead of me and I can't keep the pasture up to their standards. When this happens, there will be sections of grass that gets too tall for their delicate sensibilities. They will continue to graze the short grass and let the tall stuff grow ever taller. When this happens, I simply move them into their drylot and cut down the tall grass with a scythe and then throw it over the fence. They eat it all up then. Go figure. Buncha weirdos.
Otherwise, they're pretty much low maintenance. I keep their water buckets full and free choice minerals available whenever they need. They get hay or pasture only. I do not feed pellets as a matter of course. However, I do occasionally supply pellets to older nursing dams if they need the extra body condition during lactation.
I do not worm unless an alpaca is showing visible symptoms and I have rarely ever had to worm. I do not want to instill resistant parasites on my property and I want animals that can keep parasites in check all by themselves. The major exception would be the standard practice of monthly shots of ivermectin if you live in white tailed deer country in order to prevent meningeal worm.
Shearing is indeed more expensive when compared to sheep. Alpacas really need to be restrained on the ground in order to shear them with minimal stress. This is one of the times where I wish they were more like sheep and could be sheared by one person. I have learned how to shear myself, so that helps keep expenses lower. I've managed to get volunteers every year on shearing day so that also helps.
Alpacas really have their own way of going. When you can alter your techniques to respect this, the handling is much easier and can be done with minimal restraint. Less is better. I really recommend looking into Marty McGee Bennet's Camelidynamics techniques when working with alpacas and llamas. I can do most of my handling with the four fingered bracelet hold.
It helps to have a good vet around, which can be hard since many vets are not familiar with them. However, I have to say that most alpacas that I have encountered are pretty hardy and rarely get sick. I have only had three vet calls in the time that i have had alpacas and two of the calls involved injury.
Alpacas are similar to sheep in their copper sensitivity. However, there is growing evidence to show that alpacas probably need more copper than initially thought.
As I work on incorporating more permaculture principles into my farm, I'm looking hard myself as to how to best integrate the alpacas so I'm really glad you're asking these questions!
One of the areas that keeps coming up as a potential concern of mine is in the fencing. I currently have 5' woven wire fencing not really to keep the alpacas in, but the predators out. The alpacas do not challenge the current fencing. I have spoken to other alpaca breeders who have had issues with electric fencing. My current challenge is to better rotate my alpacas throughout the entire property, beyond their current pastures. I think if I did any electric fencing, it might have to be the net type. This would provide the visual barrier that the alpacas respect, because many breeders have reported that the alpaca fleece insulates the animal enough that it simply doesn't feel the fence. I hesitate to use any other electric fencing with them, especially where babies are concerned. I'm still researching this one myself.
I love your idea of visiting with several farms. I really encourage you to do this. I helped out at my mentor farm as well and learned so much hands-on this way. It was invaluable experience.
Above all, have fun on your journey and if I can ever be of assistance, please let me know!