Buy soon, you won't regret it! As for brands, they all offer great bikes. I personally ride a Giant, but that won out because not only is it smokin' hot, but it was at a deep discount.
Everyone says test ride the different bikes and there's a lot of truth there. However just riding around the parking lot for 15 minutes isn't going to give you a long term sense. If there's a store that let's you take a bike out for an hour or so, you'll get a better feel. However because it is a new riding style, your adjustment to the road will likely take several months, so something that feels uncomfortable now may feel perfect later.
Over the past several years the bike companies you mention all offer pretty similar bikes in both a more aggressive geometry and something more relaxed. The relaxed geometry doesn't mean it is just for comfort but these bikes have you sitting slightly more upright. In the Giant line, there is the TCR (which I ride) and the OCR (more relaxed, which my wife likes). I've had my butt kicked by people on the OCR, it isn't slowing them down.
Also, money comes into play. I'd look for a bike that offers a smoother ride - carbon (and steel) is your friend there (though not as friendly on the wallet). I'd say you'd want at least carbon seat stays & a fork, there are many bikes like that for under $1,000. For $1,300 (just guessing) and higher, you can get a full carbon bike. That's my preference, though every budget is different.
Perhaps the best approach, especially if you are unsure if you'll take to the road, is to get something low cost that you can trade up to later. For example Giant makes the OCR in carbon and aluminum - they both have the same geometry & components. The aluminum costs well under $1,000, I'd guess if you got it & liked it, you could re-sell it in a year for not much less than you bought it for. Or better yet, see if the bike shop has a 'trade up' policy. Also, fall/early winter is a good time to buy bikes, new year models are coming and the shops try to move the old stuff. Or if a shop has last year’s model on the floor, it may be discounted as well.
Now I'm not trying to push Giant, we love our bikes but I use them as an example because that's what we're familiar with. I'm sure the other brands offer a similar experience.