Where are you located? If you're already on 28s, I suspect that the road conditions are not bad enough to warrant wider tires and front suspension.

I'd go for a road bike. It will almost certainly be lighter and will allow you to ride more aerodynamically, both of which will be faster. Faster saves time and is IMO more fun.

As long as your back doesn't complain, there's nothing wrong with riding with a backpack.