Would you rather spend money on/invest in a good, quality set of speakers or headphones? The portability of the speakers may a factor here but there are many these days that can be easily moved about.
Personally, I would spend the money on the speakers. When I'm out and about, I'm mostly working or running errands; I don't tend to listen to music then.
I have never had any luck with headphones so my preference would be speakers. I usually prefer the portable ones since I can easily carry them when on the road. I wouldn't mind spending in order to get good quality ones.
I'm the odd one out: headphones. I listen to music constantly on the go, and I tend to burn out MP3 players well within their warranty. The last one lasted six months, which is good for me. Headphones which will last and have good sound quality, not tinny or thumping, are essential.
I would much rather have the speakers than the headphones. I find that the quality of the sound is so much better when I'm hearing it through some good quality speakers.
I think that I would invest in some good headphones, even though I love a good speaker. I just know that I use my headphones way more often than I use my current speaker these days.