I used to carry the spare batteries, but the portable phone charger can charge my phone AND other devices about 3 or 4 times. My ear is not tethered to my phone, so it's not a big deal to have my phone plugged into a plastic device that's about the same size as my phone. If you can't put your phone down for a few minutes at a time, then go for the spare batteries -
I don't know about that. I have a portable charger, and while it was nice to be able to charge any device, it takes more than a few minutes to get any significant charge out of it. So if you're only charging for a few minutes at a time, you're charging for a few minutes at a time several times for the rest of the day. And if you're using your phone so much it needs a recharge in the middle of the day, you're less likely to be able to set it down and let it just charge. It was nice when I got my Galaxy Nexus with a spare battery - take a minute to swap the battery and then I don't have to worry about it the rest of the day.
but then you've got to figure out when you are going to charge them. Want to spend your whole evening in the hotel charging a variety of batteries in your phone?
Assuming you get a charger that can charge the battery outside of the phone, this shouldn't take any longer than charging the portable charger.