I know some Moms successfully do both and can sustain enough of a supply to continue some nursing sessions I guess at certain times of the day. Exclusively breastfeeding every 2 hrs for 11 days of DS's life hasn't been enough to make her weight loss stop (she lost 13 oz the by 5 days pp and another 2oz 10 days pp.) So we made the decision to give her a bottle last night. And I got 5 glorious hours of sleep! Now I'm not sure yet whether I want to keep up some nursing. Anyone know how to do this?
Please don't flame.  I pumped for 4 months for DS and have vowed to never do that labor of love again.  I guess exclusive breastfeeding just isn't in the cards for my babies. 
Re: xp: bottle AND breast
You don't have to breastfeed exclusively for it to be deemed SUCCESFULL!
I think it is awesome you have been hanging in there all along. I am a part time BFer. I nursed every 2-3 hours and my DD STTN at 5 weeks. When I went back to work at 7 weeks my supply tanked. It was an emotional battle but once I realized a baby has to eat no matter what it may be...I felt better about it and I love having my boobs to myself sometimes!
It will take a while to get it where you want it, I nurse first thing in the am, and pump about 4 hours later at work. I send formula or frozen BM to my sitters where my DD eats 5oz every 3-4 hours. I can go 6 hours as well but I dont recommend it if you are really focusing on mainly feeding him BM. It averages out to about 4 feeding sessions a day for me, and maybe a comfort nurse here and there if she is fussy. If he STTN, pump before you go to bed, and if you are vigilant enough to pump in the middle of the night do that too! I usually nurse at 10pm and wake up at 7am to nurse.
Like I said, it will take some time to get into the swing of how it will work best for you, but some BM is better than none and you will do just fine!
Good luck, you can do it!
Either way, don't beat yourself up about it. You have to figure out what works for you and LO. It's hard making a decision like this when you are so exhausted and overwhelmed. If I were you, I'd nurse first, then offer a bottle if LO still seemed hungry. You can also try pumping after each feeding to try to increase your supply.
Congrats on your new LO!
DD1: allergic to eggs & dairy
c/p 4/1/11
DD2: milk and soy protein intolerant, allergic to eggs, soy, peanuts, tree nuts, sesame, bananas
MSPI Moms Check-In Blog
That seems like a lot of weight loss, and you have to do what you have to do to take care of your baby. You are feeding your LO that is all that matters. But, with that much weight loss I don't think I would let LO sleep for 5 hours. I would wake for a bottle or nursing session.
Also were you on an IV for a long time while in labor? That can cause the baby to get bloated, which can cause LO to lose some weight.