Without knowing the gravity of your new beer, it's impossible to say. If you'll be making a similar gravity beer, I think using half of your slurry will be a bit too much.
It'll probably be a little easier than washing all of it. I like washing because I can get a better idea of how much yeast I actually have. Too much yeast is too much, too little yeast is too little.
I've had poor results pitching onto a full yeast cake. That was way too much yeast. Explosive ferment, then under-attenuated. I've had better results using a quart of slurry for a 5 gallon batch, about 1.05 gravity. I've had my best results (vigorous fermentation with good attenuation) washing the yeast so I could pitch the appropriate amount.