Depends on what you'll be doing with your guitar while it's in its case.
No gig bag can offer the protection of a good, well-fitted hard case. Gig bags—pretty much all of them—fall short in two ways: stiffness and headstock impact.
Stiffness — Take an empty gig bag and make it bend in the middle. In most cases, it will be easy to flex it at least a few inches. Now put a guitar in the case and try the same thing. It's a lot harder to do, isn't it? That's because the neck of your guitar is taking pretty much all the stress. Even the vaunted Mono bags can be flexed with minimal effort.
Headstock impact — Most gig bags will deform if you bump the bag headstock-first into a wall. And with most of them, the headstock almost touches the end of the bag to begin with. And almost all of them will allow the guitar to slide lengthwise on impact, even if there's built-in neck support and a hold-down strap around the neck. In each case, the full momentum of the guitar is brought to bear on the end of the headstock.
The impact thing gets even worse if your guitar has an angled headstock. Now you have to worry about banging the headstock from behind if you turn around and bump it into something. Or if it falls over backwards. Or...
That said, gig bags vary widely. Some are just flimsy sleeves. Others are firmly padded and stiffened. The best gig bag I've seen is the Fender John Mayer, made by InCase, who also made a similar bag under their own name. Thick, closed-cell foam, tons of storage, and very hard to flex. But InCase doesn't make guitar cases anymore. If you can find one on the used market, it'll set you back between $200 and $300.
Polyfoam cases offer most of the protection of hard cases at a fraction of the weight. They're very stiff, but if the fit isn't right, your guitar can still slide forward in the case and impact the headstock. Depending on the case, they may or may not offer a size advantage over a hard case.
Here's the InCase/John Mayer gig bag and a polyfoam case...
Ultimately, you have to weigh the risk of damage against the risk of someone spotting your guitar in the back seat and making off with it. Just make sure you take a good look at the gig bag you're considering before you buy it.