Dwarf Puffers and Female bettas both have the same aggression problems when put in the tank with other fish, from what I've read, neither is more social than the other. I've read many posts on this very forum where an all female betta tank turned into a little bloodfest of territorialism.
I think a species tank of DPs is a fantastic idea, wholly achievable if you've the will and the way.
However, if I were you, I'd advise with a betta tank.
Here's why I'd get the bettas:
1) Their diet is less selective than a betta's, so it's easier to hoist them off onto a pet sitter when you travel for 5+ days.
2) Visual stimuli, you are able (even though it's technically better to have all sisters, having nonsibs in a tank together is achieveable) to get many colors and put them into the same tank to achieve a little more variety (if that's your thing, I actually think 5 or so DPs would be a cuter site... but I'm a huge DP fan).
3) If a war does break out, it's fairly easy to put a female betta into a smaller bowl and maintain their health, wereas having to seperate out 5 dps into smaller bowls will cause a whole mess of issues, since they're so messy they'll each need 5g each, and since they aren't community fish, none of them can remain in the 29g (unless you want the most spoiled, unseen DP, ever). You can also leave a female betta in the tank and fill with community fish, if she's of the disposition... that's generally not an option with DPs.
Of course, those are my opinions.

Both fish are valid options, if you're willing to see to their needs. If a DP tank is on your personal, "omg want" list, I'd say go for it. It's doable.