I rather Laurie stays a pet. Don't get me wrong, I treat my pets with a lot of love and affection, but I don't want her as MC's vamp child. I honestly think turning Carmen would be more interesting than turning Laurie, if MC is going to turn anyone. I am sure Laurie's jealousy will be so much fun to watch. Just thinking about the possible cat fights between Laurie and Carmen makes me drool.
But more than that, while most of us would agree that being a vampire would be awesome, from the point of view of the characters it is almost never portrayed as something good to happen to you, unless you are the standard evil vamp "we are the real rulers of earth, god among humans and all shall bow to us" type of character. So MC turning his long life childhood friend would be kind of hurting her. He already hurt her when he enthrall her just because he was a selfish douche that could not let her go and have a happy life.
Only way in which it would make sense is if MC and Laurie becomes a couple, as otherwise there is a clear position of power in that relationship. But as I don't care at all about Laurie as a love interest, she can remain what she is.
Turning Carmen, on the other hand, would allow for a totally different interaction. Specially, I would love to turn her when she is still angry and hate vampires. Making her slowly realize MC is not an evil monster, just a horny one. And slowly realizing that all she knew about vampires was wrong and that she, while now a monster, she can still do good and potentially live a happy life. It will also allow for Sharon to become a bit of jealous.
Now imagine how fun it would be when sire appears, with a renewed interest in MC she was not expecting, and starts making life a nice hell for MC, creating all sort of conflicts for him and his friends, and slowly seducing him while playing the part of a mischievous villain, until a new real villain appears.
"S: She turned you and left without teaching you or explaining anything. Why are you still playing her games?
MC: Sharon, she is my vampire mother. I need to understand why she turned me.
S: Is that or is it how it always go with you? See a hot girl and can't stop trying to get into her panties as you did with Carmen.
C: Hey, I did not ask to be turned, you guys did it to me against my will.
S: Did not stop you to bang him the minute you had the chance as a horny teenager, slut.
L: At least both of you got his attention, I have known him all his life and he treats me like a pet.
MC: Laurie, I am only doing what is best for you, your attraction is not real.
L: What would you know, you dense son of a bitch, everybody but you knew I was in love with you when we were humans, you stupid asshole. Just ask the readers and they will tell you.
Merrick: Hahaha see kid, you cannot just be a slut and hope they will all be happy about it. We nos maybe as ugly as a pus infected alien, but at least we know how to treat our women."