At a bike cross road

Road bike or Triathlon bike?  Depends on your priorities.  If you prefer road riding as opposed to triathlon then a road bike is a probably the best option.  A road bike is more practical in the sense that you can make adjustments to improve its "usability" in the sport of triathlon (install clip-on aero bars, lower stem, adjust saddle).  The speed benefits you'd get from a Tri specific bike are very minimal.

Buy new bike or just frame?  In most cases it is much cheaper to buy a new bike than it is to build one from the frame up.  On the other hand, if you already have a good parts group then building one may be a good option IF you find a good deal on a frame & fork.  If you chose this option make sure you know what frame size you need otherwise you're back to square one with a mis-sized bike. 

Better to have a bike too small or too large?  Neither is ideal, but its typically easier to fit someone to a bike that's too big than it is to one that's too small, but you already know that a bike that's too large is uncomfortable and there's a limit to what you can do to adjust it to fit.  If you're going to be riding for years to come then it's important that you get a bike that fits perfectly.  You wouldn't settle for a pair of running shoes that were too small or too big, would you?

Brands?  Pretty much all of the big name companies are the same as far as quality are concerned.  Some offer better "bang for the buck", like Felt or QR.  Felt has a great reputation for quality and price.  QR has quality issues and the company is in financial trouble, but they too offer a high-tech bike for little money.  Otherwise, Trek, Giant, Specialized, Cervelo, Scott, etc.....they're all pretty much the same.

Good luck!

If you only want to do triathlons, get a tri bike, by all mans.  You can train on a tri bike as well as to race on it.  A tri bike is not only constructed for speed but also to minimize the strain on your legs to leave them fresher for the run afterwards.

You could buy a tri frame and build it up with the components on hand if you know how to disassemble from the Giant and onto the new frame.  You could pay your local bike shop to do the swith for you, which would cut into any savings you might have.  Or, you could buy a new tri bike, sell your present road bike or trade it in on another smaller road bike.   

There is no "better" between a too small and a too large bike.  Both are equal bad!  Stop riding that bike now.  

AS the previous poster states, most of the big name brands are pretty much equal in  quality.  Personally, being Canadian, I prefer Guru, Marinoni, Argon18, and Cervelo but KHS, Felt, Javelin, Giant, Specilized, Trek and Cannondale also offer good quality bikes in all price ranges.  Shop around and compare.

People do Ironman on tri bikes. If the bike fits, it is comfortable. I do all well most of my long rides on my tri bike. You can do century rides on a tri bike. Seems you like riding in areo which means the best fitting bike for riding in aero is a tri bike.

Key thing is to get a good fit on next bike. Most tri bikes are generic ie really mean they are geared for men. There are a few tri bikes for women but far fewer.

I bought a too long men's road bike...stupid mistake as it was my 3rd bike. I since have bought a women's road bike which fits me well so I'm riding that outside in the messy roads and have my tri bike on my trainer.

What brands we all like our own bikes. Key is fit fit fit..which fits you best? If you are maybe 5'4" or taller you can fit into many or most tri bikes. Shorter than that there are fewer options.

I'm planning on doing a 12 hour road race on my tri bike next year. Supporting my upper body on aero bars is way more comfortable than sitting more upright. I did a hilly ride last year and was on my tri bike 8 hours and wasn't sore the next day...fatigued yep but I thought that shows that my bike really fits me well.