[Mystic Meloetta (Aria)]

Pretty good investment for a rare Pokemon that will be sought after by the rest of the community for their collections. You can trade it and sell it for a lot if you want, but Mystic Meloetta (Aria) is not particularly powerful for use in battle with its Psychic/Normal typing and inability to One-hit knockout enemies. Dark Meloetta (Aria) can if you teach it Giga Impact, as with Same-Type Attack Bonus and Dark multipliers it does over 400 damage on enemies without resistance or immunity to Normal-type attacks. Myself I would recommend Dark Hoopa (Unbound) if you want a donation Pokemon that's good in battle, as you can teach it Fling to do over 400 damage and fewer enemies are resistant to Dark, and Hoopa (Unbound)'s Dark/Psychic typing is better defensively overall I find as Dark is immune to Psychic and resistant to Fighting which is really useful in the sidequests.  

 

It's also worth noting that this is a 2nd edition, as 1st edition for donation Pokemon has passed. That may come into play depending on who you trade, in which case earlier editions may be preferred. Either way, it should be worth a fair amount in terms of trading and auctioning value, but if you want to maximize its value then a Shiny would be better. I bought this mainly for my Mystic collection, and if you're collecting a certain special type then all donation Pokemon of the desired speciality are great.

 

 

[25x Mystery Boxes]

In this batch of 25 boxes I got no Beast Balls/Vortex Balls or unique Rotom forms. You need to get a "Top Tier Box" for those, which has a low chance of dropping. Otherwise, you can expect just avatars, Mega Stones, a few tens of thousands of Pokedollars, and the occasional Normal Rotom form in a box.

 

I purchased another batch of 12 boxes afterwards and fortunately got a Top Tier Box with 10 Vortex Balls and a unique Rotom form, but out of 37 boxes that was the only box I'd have paid anything for if I knew the contents beforehand. Everything in my $30 of Mystery Boxes combined would've been about the same value as a single $10 donation Pokemon considering current rates on Pokebay, so you're definitely better off purchasing donation Pokemon, then you can sell them on Pokebay and buy Beast/Vortex Balls/Rotom forms at a more efficient rate than buying Mystery Boxes (about triple the efficiency in my case). BonelessCow's Mystery Box opening on YouTube has about the same rates as I did too, so in general I wouldn't recommend this at all. 

 

tl;dr Don't buy Mystery Boxes. You're way better off buying donation Pokemon if you want to support the game and your account.