Hack-a-thons are kind of hit and miss. You get a stronger chance to make a good impression on two or three guys.
MeetUps - you have the whole community which knows each other. You can for example say that you're interested in picking up contract work or pairing with anyone anytime. At a MeetUp, someone will say..."I know Dave is looking for a person to pair with as he's never trained anyone and he's just been promoted". That stuff is hard to come by at a Hack-a-thon and to be frank, great programers at a hack-a-thon have difficulty even finding small features for new dev's to develop.
I remember my first hack-a-thon I worked with a great team, but I ended up working on the "About us" page, because I really couldn't contribute as much as I would have liked.
Stay with it - MeetUps are ten times better for future employment than Hack-a-thons in my humble opinion. That's not to say that a Hack-a-thon couldn't be end up creating an opportunity, but the circle is soooo much smaller.