Trust me, Go for chickens.
It seems like a bad spot for f2p training, but it really is nice xp+cash for your level.
1hitting 3's on these...killing 10+ a minute..
I stand by what I said in a similar post to this one in that you should definitely do soulwars since it's clean, fast, and a lot of fun if playing with a couple friends but if you're f2p then I'd suggest falador cows cause they're VERY close to a bank so you can train along side of keeping the hides, bones and/or meat at a reasonable xp/hour rate.
Chickens. I got my pure from 1 to 50 strength in 3 days. I got something like 450k from feathers alone. It makes good starting cash. Another way is to kill cows and bank the hides.