should i train today or start fresh tomorrow?

Train, you'll feel better after you train even a little is better than none. Even if you are sick some training is good unless you are too sick to stand up and then you should rest. But if you can stand up, you can train.

Gaining muscle is gaining weight too. Muscle is heavier than the fat you put on from sitting on your butt.

I'd have to disagree with Liondancer on this one. I wouldn't train unless I wasn't in a good condition to do so. I would not train or go to the gym if I felt that I was getting sick. Aside from being courteous and not spreading germs around, your body needs the energy to recover and fight sickness. A strenuous workout can compromise your immune defenses and ensure that you get sick. In short, not only have you put your progression on hiatus, you'd compromise your training and gains which you've worked hard for. Personally, I get less than the average recommended daily amount of sleep but train if all I had was a little sleep deprivation; never when I am sick. Having said that, if you're looking to gain mass, rest is essential. Getting at least 7 or more hours of sleep is ideal. Also training twice a day isn't going to help you build mass. Have a single intense workout a few times a week can be more beneficial to you for hypertrophy.

Always train. If you have time, train. Do as much as you can. Wanting to "start fresh" the next day is just a lazy way out of training. Trust me, I try the same thing.