Navigating traffic.
Actually, I've been commuting between the south end and north end for quite some time and I-5 actually flows very well, even when its stop and go traffic. You can normally figure on about 45 minutes to go from Renton to Everett - it can definitely be more, sometimes less. Both freeways are usually pretty good between 9am and 3pm. you'll obviously have people with you, so you can use the car pool lanes (the lanes with the diamonds).
With that in mind, if time is important - take 405 and stay in the car pool lane. We also have the express lanes on I-5 that run between downtown Seattle and north Seattle. 3Am to @Noon, they are open going Southbound. 2PM to about 10PM they are open going Northbound. If you do take I-5, and are towing, just south of area where Safeco Field and Seahawk Stadium are you will hit an elevated roadway section that always makes me cringe when towing. You get some bucking due to the up/down condition of the highway. I-5 north bound in the afternoon can get really bad north of Everett since there isnt an alternative. Just try to go through before 3 pm.
I'm going to stop with that - I'd say dont worry about it too much - since you'll be able to use the carpool lanes, you shouldnt have any problem no matter which way you go.