Well, easy question first. . . The OVA was made 
before the TV series. The order you watch them in 
(TV first, or OVA first) really doesn't matter, 
but the episdes within each one should be watched 
in sequential order for the best viewing.

As to the cels. . . OVA cels are always more 
expensive than equivalent TV cels because the OVA 
not only has nicer artwork, but it is also older 
than and was MUCH shorter than the TV series. OVA 
cels more often come with the matching sketches 
since they were mostly released before AIC 
stopped letting their sketches out. Larva is 
possibly the most expensive character to collect 
from the show (without mask is generally more 
valuable than with mask), but really nice cels of 
the shinma (especially in their true shinma 
forms) are hardest to find. Further than that is 
tough to say since lately more than ever, Miyu 
cel prices are all over the map. o_0;

But like anything with cels, pay what you're 
willing to pay and use your head when doing it. 
There are more than a few Miyu cels hanging 
around collecting dust on dealer's sites because 
they're VERY overpriced. (In my always so humble 
opinion.) And also some collecting dust, not from 
overpricing, but because I can't afford them. ^_~ 
I can say there's little chance I'd pay 48000yen 
for *ANY* profile shot, OVA or not. But there are 
a few Miyu cels out there (both OVA and TV) for 
which I would fall over myself to be able to pay 
only 48000yen to get them. The value of any cel 
depends on the beholder.

Prior to my commenting on the manga, understand I 
haven't *actually* read lots of it. I've read 
some of the Studio Ironcat translation, and I 
have the original Japanese copies. (Before you 
ask - no. I can't read Japanese.) The Larva that 
you see in the OVA seems much more like the manga 
Larva than the TV one. (He's very chatty in the 
TV series!) Remember, the whole point of the mask 
was that his face and voice were sealed away as a 
punishment when he let himself become a slave to 
Miyu.

Anyone who's read further in the manga than I 
have is welcome to correct me. ^_^  Better yet, 
sell me more of the bound Studio Ironcat editions 
that my disfunctional bookstore can't seem to 
order. ~_~;

Then there's Vampire Yui. . . I don't know a lot 
about her. Her manga is drawn/written by the same 
person who does Miyu, and some drawings are done 
of her and Miyu together. . . But I think she's 
in a completely separate story. Lacking either 
anime or English translations of the manga, I 
haven't bothered with it. But I do have some 
pictures of her in one of my Miyu artbooks.

Many Sharp Smiles,
--Drac