Full disclosure: I have played Band Hero for 10+ hours with friends, but have not yet played Guitar Hero 5. Information on the latter has been gleaned from articles and other reviews.
I've played through every major iteration of Guitar Hero from the original to Guitar Hero: World Tour, and I thought Band Hero might be a good change of pace: more well-known pop songs to bring in the people who aren't fans of the more classic content of the series.
Unfortunately, the developers have chosen to take the track list length back to the GH2 days: only 65 songs are included, whereas GH3 had 73, GHWT 86, and GH5 85. It might not look like a huge difference by the numbers, but that's almost 25% less content, and you definitely notice when loading up a playlist. I understand they want to sell downloadable content, but chopping the number of included tracks while only bringing minor new gameplay elements to the table is somewhat disappointing. To bring the track numbers for BH up to GH5 levels, you'd have to get 20 more tracks at $2 apiece, so add $40 to the purchase price of BH for a true comparison of the two.
Of course, I could tolerate less included songs if they were all solid, but in my opinion, that's not the case. While I can tolerate a small amount of really cheesy, outdated 80s pop music, they really got the low points here: Culture Club, Poison, Devo, The Go-Go's, etc. And, as one would expect, many of the pop songs get very repetitious while playing on the guitars and drums. I've gotten bored on the drums a few times during GHWT, but probably one song in three on BH makes me zone out while awaiting the chorus or bridge.
GH5 has all the new interfaces and gameplay features (Party Mode, band moments) with 20 more tracks and (in my opinion) better songs. You can just buy both and import from one to the other (you can import to either from GHWT), but I'm looking to choose between the two, and Guitar Hero 5 seems to be the superior game.