I feel your pain. I was exactly where you are now just a short while ago.
Ultimately, I decided on the Optima. However, I think that the Meg is also an excellent unit. It has some very robust engineering, and it is solidly built. I just decided to go the other way.
My main concern about the Meg was the long hoses on the loop. I dive a lot of wrecks. The hoses behind the head keep setting off the entrapment alarm in my head. When I would bring this up, people would just tell me to use the larger tanks that project farther above the head as a prevention and blocker for the hoses. That just did not cut it for me.
My second concern was eliminating caustic cocktails. I really think I moved in favor of the Extend Air cartridges after actually watching somebody pack a Meg canister with Sorb. The dust was blowing all over the place, and I felt a burn in my throat from (even from pretty far away) when the wind carried some of it in my direction (even though I thought I was standing out of the breeze). Then, I saw the person who was packing the Meg start to smack the canister with the side of a table to tamp down the Sorb. For me, this episode finally tipped the scale in the direction of the Optima over the Meg.
I am in the process of training on my Optima as we speak. There are certainly some things about it I would change if I could. However, I like the basics of the design. Dive Rite (yes, I know it's really Farallon sort of) has excellent customer service, and they tend to have fast turnaround should you ever need to have servicing done on your unit. Since I don't have any actual experience with Meg service, I won't comment; that wouldn't be fair. However, I would ask around about this from others in the know, factoring the answer into your decision-making process.
I think that lots of rebreathers seem to have some things both positive and those that everyone wish could be changed. So, it will be hard to find one that has everything you want without any negatives. That just seems to be the way it is.
I am happy that I elected to get an Optima. However, I wouldn't turn down a free Meg.
By the way, you are not restricted as much as you think with the Optima regarding tanks. You can use smaller tanks (such as 19 cubic footers, etc.). However, you will then have to add weight. The designers use the bigger tanks to produce the same buoyancy with a greater volume of onboard gas. This is something I also like over other rebreathers. After all, they will all pretty much have problems at one time or another. Having extra gas for the just in case scenarios seems like a good thing to me.