Some Medicare Advantage (MA) plans have a high Maximum Out Of Pocket (MOOP) to offset the low premium. With Medigap Plan G, your medical MOOP is limited to the Part B deductible of $166 in 2016. With Medigap you do also have to choose a Part D drug plan that may have additional out of pocket costs.
If you do choose MA, you have a one year grace period to come back to Original Medicare and choose a Medigap. This is known as a Special Enrollment Period (SEP). After that time expires, you can still come back to Original Medicare but would be subject to medical underwriting for the Medigap.