We were at Skipjack for many years and have visited Great Oak on several occassions. I can tell you that while SJC has some negatives we do prefer it over any other marina on the Eastern Shore within easy reach of I-95. The negatives for SJC are that its 7 miles by water from the Chesapeake and the service department is expensive. The pros are that the Sassy is (IMO) the prettiest river on the Upper Bay, their are many places on the river to anchor out safely for the day or the night, fresh water (most years!), SJC has the cleanest air conditioned and private bathrooms on the Bay, the grounds are kept very clean and maintained and the staff is helpful. If you have kids there is the pool, horseshoes, tennis, basketball and a dog walk area.
The pros for Great Oak (IMO) are that its close to the bay by water and its a well kept marina. The cons (IMO) is that its too big, the Creek is a party scene every weekend (and not necessarily "kid friendly") and the entrance to the creek can be a challenge at times.
Let me know if you have any more questions about SJC and I'll be glad to help you.