Panasonic 20 f1.7 or flash for 18th

[Deadhead]

My partner's son has asked me to take photographs at his upcoming 18th birthday party. Its being held in a pub; there'll be a hundred or so people with karaoke, dancing and so on and he's going to want pictures of the action and most of the people.
I've got a Panasonic G1 with the 14-45 and the 45-200 lenses, neither of which are probably ideal for this job.
So, here's the question. If I only buy one thing to help with this project; should it be a] the faster 20mm lens, or b] a dedicated flash to augment to one built into the G1?
All views appreciated. Thanks for reading

 

[Danny_Two]

The 20mm is amazingly bright, but the DoF is tiny at max aperture. I still think it would perform well with the right ISO, and as you say, you have a flash built in if you need it.

 

[gingerweasel]

I'd go for the 20mm and pick up a an Olympus T20 flash from ebay. I ordered one last week 2nd hand for less than £22.

 

[dazzajl]

Clearly it's your hard earned John but I'd go for the lens. On the night it should let you get good shots combined with a medium/high ISO. Shooting that way is also more fun than wandering round blinding folks with a flash.

After the event the lens will probably be more useful than the flash too.

 

[grum]

I've got an external flash for the G1 and the 20mm, and I'd definitely go for the 20mm - but then I prefer the natural light shots. The DOF isn't that tiny at 1.7 if you are a little way away.

The 20mm is a fantastic lens anyway.