I haven't had first hand experience but one of my friends had alpacas and from watching what she went through with them I'd say buy the roving!
There's so much to caring for a living thing that you don't think about when you first get that furry little (in this case, not so little) creature. Food and general care is one thing but I can remember several middle of the night emergencies with sick alpacas and many midnight visits from the vet.
Some of her alpacas mated every now and then (oops, how did he get in that pen???) and there was the pregnancy and birth to deal with. She was happy to see them go when she finally sold the last one.
My friend didn't even knit from her alpacas. Someone she knew came and sheared them and cleaned, carded and spun wool for her. Just the alpaca care alone seemed like a full time job.
I know there are probably some of you who would disagree and like I said, they weren't mine so my opinion is second hand.
I have been told that you can spin your own yarn from alpaca bunnies and they seem to require a lot less care than alpacas.