We haven't driven to WDW in 10 years but unless there is some big event in Jacksonville on a Saturday morning you should be good.
When I drove we usually went thru around 10:00 am on Saturday and there were no issues.
When you are hitting it, I'd probably drive straight through, unless I was just in the mood to pick up 17 and 19 down through the forest.
I like that route, but even so, I'm not sure I'd want to do it on a December Saturday.
So stay on 95, I think.
One thing to consider in the 95 vs. 295 discussion is if you want to get gas near here. Jacksonville has the cheapest prices in Florida in general and off 295 vs. 95 in particular. There are many good easy on/off exits on 295 to get the cheapest gas at. I usually fill up in NJ, VA, SC, and Jacksonville area on the way to Orlando. Gas was $2.47 for regular around the 5th exit down on Monday. Sorry to not be exact, but it's the first exit with multiple gas stations listed on the services at this exit signs.
I prefer I-295 east (heading south) vs. I-95 straight thru town. There is construction going on in downtown and its a narrow curving route right now. I find 295 to be an "easier" drive personally.