Kx intro question
I have benefited from both, but my first choice would be a better lens. And by better, I simply mean a faster lens.
The first lens I bought besides the kit lenses was the A50/1.7. I'm not really a big fan of that focal length on the APS-C sensor, but I have gotten some good results from it indoors. And the good news is that it is relatively inexpensive -- under $100 usually. Here are a few examples which make use of wider apertures in somewhat-low light.
Later I added the DA70 Ltd, which is hands down my favorite lens. Here is an example from it using a wide aperture.
And most recently I've added the DA35/2.4. This one may be the one I'd recommend most given its versatility and the fact that it is an autofocus lens.
And just to illustrate what a bounced flash can do, here is one which used the Metz 48 along with the DA70.
The benefit of the flash is, of course, that you don't have to use higher ISOs. But honestly, I find that if I get the exposure correct at the time of shooting (as opposed to having to make corrections in PP), even ISO 6400 is usable for simple 4x6 prints. Here is one I shot a few weeks ago at a zoo. While technically the monkey was in an outdoor setting, I was shooting indoors, handheld, looking through thick glass, and using the rather slow 55-300mm kit lens. I think what this illustrates is that you can get good results indoors with the kit lenses without going out and spending a lot of money on a flash or lenses (even though those are fun to have).
-- Joe S.
'The laws of nature are but the mathematical thoughts of God.' ~ Euclid