I guess for mashup Acid and Live are both cool. They will match the acapella to the beat or vice versa but it ain't magic. You have to "tell" the program were are the beats and things like that before. Some mash dj prefer the hands on approche of the plates... Which one you choose depends on how you like to work. Acid look just like any other DAW (like your sonar3 for exemple) but Live is definitly in a category of is own. Live is more intuitive (but people would say exactly the oposite tho...). Me I prefer Live. You can use it live tho (arg... stupid name...). The algorith for stretching sound sounds ok in Acid but in Live they are a bit better.
I don't know sonar3 so I can't tell you how to do it. I can tell how people use to do stretching tho and that you can use. You have to know the perfect bpm of your sample (ex. 90) and the desired bpm (80). Now you want to know how much stretching to aply in %, so you multiply 80 by 100 and divise that by 90. 88.88%. Your final sample shoud be stretch 88.88% the duration of the original. Now you can stretch it by this value in many program, I would be surprised if sonar don't let you do that... Hope it help a bit.