I agree with what funcinderella said..
Any other time of the year, I would probably tell you to do value as it is nice to be onsite but in Jan, the parks close early and the crowds are low so you won't have to really utilize a lot of the onsite perks such as extra magic hours and easy access to and from the resort when going back for naps.
If it was crowded, real hot, and parks were open past 8pm I would tell you utilize onsite hotels as ds would have to nap everyday, even if he normally doesn't.
If you do not have a AP though you will have to pay for parking everyday if you stay offsite, so take that into consideration. It would probably even out to all the darn resort taxes you pay anyway.
I am not sure the dates you are going but another option is, you could rent points from a DVC member for a studio at Old Key West. We did this when dd was 3 and it was wonderful. They usually charge $10 a point so for you it would be $80 a night during the weekdays (weekends are higher) but that includes all taxes etc... There is a sink, fridge, microwave and you can park your rental car right outside your door. Personally I can not stand the buses, it just takes forever sometimes so we liked just hopping in the car and parking free and coming and going when we wanted.
But then again, even OKW doesn't have the pool, hot tub etc... that you and dh could enjoy so I would probably stay offsite and enjoy some time together as a couple while DS sleeps. Values don't even have a balcony so you are sorta stuck in the room. If he doesn't nap during the day, he WILL be in bed early and you will be BORED in the hotel room 5 nights in a row.
Before you know it DS will be older and will want to be at the parks all day and night and you and DH will have NO down time and will be asleep from exhaustion before him

THAT is when you stay onsite.