Even though IPB v1.3 is so old now, it still beats phpBB2, and even phpBB3 on a free host. So, I rather use InvisionFree than a freely hosted copy of phpBB3, simply because I love IPB that much.
The biggest things that make IPB better than phpBB imo are updates and MODs. Installing MODs takes too much time on phpBB when you should be advertising your forum or being active on it. Updates are almost worse...conflicts just suck, simple as that.
You also have to look at IPB's skins as well. IPB is the most customizable software imo, adapting to every need without too many issues. I know phpBB can be adapted as well, but its harder.
For me IPB is the winnamen, unbeatable by any other software. However, if you have no money to spend on IPB, then phpBB is the next choice in line -happy-