I have a mountain bike and used it in my first triathlon without slick on. Now I'd like to get into doing triathlons just for fun, not competitively. I'd also like to get into doing cyclocross. Here's my dilemma...should I purchase a road bike and just use my Gary Fisher mountain bike for cyclocross or should I buy a cyclocross bike and use it for Triathlons? Any suggestions? Also what are the top brands for both road bikes and cyclocross bikes for a 5' female and cost range around 1K?
It has to depend what your priority will be
If the triathlon's are going to be a priority consider spending all of your money on a road bike and add some clip on aerobars. You could then spend some time adapting your MTB for Cyclocross racing which takes a little effort but can lead to good rewards. The joy of cx is that you can ride whatever you like and have just as much fun.
If your priority is going to be triathlon events AND doing better at Cyclocross you would be better suited to spending the money on a nice cx bike. Make sure it has a 46/48t outer chainset which will allow you a big spread of gears and your only real adaptations required will be the tires.
If you do choose to buy a Cyclocross bike the big debate at present is whether you ride discs or canti's. Disc's haven't made big strides into European Cyclocross as yet and my opinion is that when Sven Nys starts riding them-That's when it's time to swap from canti's.
A cross bike will do a little bit of everything fairly well. A triathlon bike will only do time trials. You're also going to have more options at the $1k range with a cross bike. Slim pickins at that price point for a tri bike. Get some clip on aero bars and get a cross bike. If down the road you really get eaten up with triathlon, and get a dedicated tri bike.
If you have the budget for only one more bicycle you cannot go wrong with a cyclocross bike, you will be able to race CX in the fall (typically after triathlon season) and you can get some fenders and slick tires for road rides in winter/spring and take the fenders off when the weather turns nice for Triathlon season. You can get clip on aero bars if you really want to, but since your planning the triathlons as fun events, there is no need for aero bars or a tri specific bike.
I use my CX bike year round, either with CX tires during cross season or fenders/slicks during rainy season and take the fenders off during the summer months.
You can get by with a MTB for cross races (I see people all the time with MTBs at cross races), but it will be heavier and harder to carry up the run ups and over the barriers.
Take a look at Kona - 'Jake' or 'Jake the Snake' for a good CX bike.
Keep that mountain bike and try an XTerra (offroad) triathlon, lots of fun!