I think it's a matter of personal preference.

I've been running my own Mastodon instance for several months now, and I've enjoyed it. I don't have to rely on someone else, either, which is nice. I'm in control of everything on that instance.

As for Lemmy, I just started my own instance today, and am currently writing you from it. What made me decide to setup my own instance was some performance issues I was seeing with Lemmy.world, although that might have been an UI problem. Anyway, I enjoy doing this stuff, so I'm running my own instance for the sake of doing it.

On the flip side, it's more expensive and time consuming, and I'm the one who has to worry about backing up data, etc. Like I said, though, I enjoy doing it, so it's no big deal.

how much does your mastadon instance cost per month? what do you expect for lemmy?

I'm using Linode, and their prices are publicly available.

https://www.linode.com/pricing/

For Mastodon, I'm using the Linode 4 GB while the Lemmy server runs on the Linode 2 GB option. Both are under the Shared CPU pricing -- not dedicated.