Mountain Biking - Size..help

I'm looking for a light x-c bike but seem to be right in between sizes. At 5'8" and a 30" inseam I fall right between a 16 and an 18 inch frame. My riding style is a little upright and I'm not real keen on a streched out riding feel but do ride aggressively and fast. In general is it better to ride a little smaller or bigger frame set. I'm looking for a light hard tail race type bike and won't be using it for big drops..just tight, fast, rough, up and down single track. I do ride a medium size FS bike that is just a bit big.
Any help or suggestions much appreciated! 16 or 18?
Thanks,
Gary

does the brand you're looking at make 17 inch frames? i remember giant used to, it may be worth your while to try that size if available.

your description seems to indicate that you'd be more comfortable on a smaller frame rather than a larger one, but even 16 sounds a little too small. if you find the cockpit too small, you *could* get a longer stem, but that tends to make your steering less responsive and may be counterproductive to riding tight/fast singletrack. highly recommend you try finding a 17 and try that out first

I've always felt that for a mountain bike, if you are between the two sizes, it's better to go with the smaller of the two.

A smaller bike is more manueverable. A smaller frame is stronger. A smaller frame is safer for Richard and the Twins. A smaller frame feels better when jumping.

I "should" be on a Large (i.e. 18" or 19") frame, but have always felt too stretched out. I am more comfortable on a Medium frame. However, that's just me. You might be different.

Try to test ride a bike that is available in both sizes, so you can immediately switch back and forth.

Thanks for the reply! The 16 I was looking at has a tt length of 21.5 with a what appears to be about a 110-120 stem so I decided on the 16 as there wasn't a 17. I usually wind up with a 90mm or less size stem on the 18"/medium bikes I ride which makes the steering a bit quick. I'm going back to a superlight ht for the tight stuff which hopefully should make the smaller16 about perfect as you suggested...Guess I'll see..it's ordered...can't wait.
Thanks again!
Gary

i'm 5'11" but with short legs. i always read that you stand over the frame on tip-toes in front of saddle, there should be about 3 inches between you & top tube. so i got me a 17" muni mula.

also look at pictures of bikes. some are measured to top of top tube, or top of seat tube, & some frames have extended seat tubes.

other things to consider are stem length / rise, & crank length.