I'm currently going through the downsides of a 12-mo lease. I don't have extensive property management experience, but I've been doing it since 2015. I'm in CA, which I think is its own beast. Laws are set up different, and stricter. Your situation will probably be different, so take what I have to say for what it's worth.
Before my most recent tenant, I was a solid advocate of 12-mo leases. It gave the advantage of locking down a tenant so you didn't have the worry about having to regularly re-rent the property should the tenant give notice. Compared to my current tenant, the past ones (who seemed pretty good at the time), now seem like perfect-angel unicorns. They were clean, paid rent on time, or early, and never made a fuss. If I had to make repairs or updates to the property they'd happily oblige and even went out of their way. I think they knew that I was only trying to make things better for them.
My current one. Wow... I thought I had screened well, but somehow she got through. Honestly, I think I was in a little rush. I needed to get it rented so that the purchase of another property could go through. She has been nothing but bad news. I did a thorough financial screening and called her past landlords. They said only good things, even the older ones. I understand the prior landlord lying in order to get her out, but why would the older ones also lie? I'm still thinking on that one.
Six months in, she has paid rent on time about half the time. She allows her pets to destroy the property. Her illegal guests disturb and endanger the neighbors. She doesn't allow me entry to make repairs, even though it's my right. Basically a nightmare. I'm counting down the days until she is someone else's problem. I've consulted multiple attorneys. When I told them she was on a 12-mo, they said to never do a 12-mo. Even though she chronically pays late, is dirty, is destroying the property and disturbs the neighborhood, the attorneys said that there wasn't much I could do, unless I really wanted to fight her and try to evict. In order to to do so I'd have to forgo multiple months of rent until it gets resolved (you can't evict and accept rent) and there isn't even a strong guarantee that I would win (CA is a tenant-siding state). So my only recourse is to hold my breath and wait six more months until she's gone. Even regarding the denial of entry thing in order to make repairs. The attorneys said that, yes, it is my right as landlord, but there's nothing to keep her from claiming that I stole property when I entered. They said I would literally need to go in there with another person and film the entire time just so I could prove that I didn't steal something.
That being said, some things will probably be different in your state, and your city. I'd ask the local attorneys what the law is like and how bad it would be to end up with a terrible tenant, then weigh the risks/rewards of shorter/longer leases. A M2M may be a little inconvenient now, but at least it gives you a solid out in case things go south.
Hope my misfortune can help you make a better and more informed decision. Best of luck.