I'm told that if you check statistics on AW you'll find that doggy style comes out tops (I've not checked myself). A WG told me that this is down to three reasons...
1. She doesn't have to look at your ugly mush.
2. She has more control and can make you come quicker.
3. She doesn't have to pull sex faces and so can think about things like what to pick up from ASDA on the way home.
That would be the old "statistics show..." or the equally theoretical "studies show..."?
How about "educated guesses" show... 
Non-Romanians:
Doggy
1. She doesn't have to let you see how bored she is
2. Saves her opening her legs (unless she a fattie)
3. Easy to play with her mobile phone or re-adjust the 'time up' setting
4. Less likelihood of any fanny-gunk on the sheets
Romanians:
A)Missionary or B)Cowgirl
1. Let's you admire her make-up while staring glassily at the ceiling
2. Leaves both hands free for playing with her mobile phone, making double bookings, asking Sergei for her passport back etcetc
3. Lets her enjoy her second favourite pastime: lying on her back and doing fuck all (the first one being just lying)
B: (Usually the stroppier breed of Romanian)
1) Let's her decide when you've 'finished' and make a quick getaway to the bathroom
2) Makes her feel masterful and in control (after being abused and sat on by Sergei)
3) Allows her to look down on you while trying to remember the English word for 'scum'
4) Reduces inbred paranoia about punter removing condom, shoving it in her arse without paying extra, stealing her knock-off Nokia phone etc etc
Dear prossies: you're paid to fuck, nicely if possible,
not have 'favourite' positions (unless
you are paying!