I use a JD MX 10, slewing, semi mount in my orchard. I like the control it affords for both height and slewing, while mowing. Took me a while to get use to it, but I can mow in rows down my orchard and get close to the trees without hitting them or leaving strips of grass between them. Pulling it with a Hesston 100 90 DT, so PLENTY of hp.
Also, backing up with the mower is a lot more precise than a pull-behind would be. Maintenance is not to hard, even replacing gearbox spindles is doable by a person with moderate mechanic's skills. As for storage, I put it in a wood shed bay, sideways (see picture, that is it in the middle bay of the wood shed). I lift it using chains on the FEL. I can lift it the long way and set it down on this raised floor. For longer distance transport, I would use a snow mobile trailer, loading the mower the same way, ie lengthwise. Truth is, the mower has never left the property since I bought it 10 years ago.
I have looked at bat wings, but for mowing an orchard, there is just too much going on at once with those. Job number one is getting the grass cut without hurting the trees...... Job number 2 is getting it done some time this year!
Now a slewing flail mower would be sweet!. The problem is, neither the JD MX 10 or a slewing 10' flail are cheap, at all!
