It all depends on what experience you're looking for, what time of year you're there, and what the weather's doing.
If you want the experience of sailing
, and sailing on big water, with the ability to duck inside the reef to catch a mooring buoy at one of the reefs or sail in the more protected waters of ther Hawk Channel, then you'll want to be on the outside. If you're OK with a lot of motoring, but enjoy the ability to find secluded anchorages amongst the mangroves, then go on the inside.
If you want to go snorkeling, stay on the outside.
If you like fly fishing, stay on the inside.
If the weather is nice and you have a nice breeze from anywhere but south, stay on the outside.
If the weather is nasty and blowing hard out of the east or south, stay on the inside.
Either way, make sure you have a Seatow or TowBoat membership in good standing (cheap insurance in case something breaks), current charts
and at least a decent handheld GPS
, and a VHF
If you stay on the outside, and you like snorkeling or diving, don't miss tying up to the mooring buoys at the reefs.....get a Top Spot chart and it will show you where they are. Carysfort Reef at the north end of Key Largo has a big reef area for snorkeling and a steep drop off on the ocean side...lots of sea life there. Also there's a cool old lighthouse built on the reef. Dry Rocks has the Christ of the Abyss statue. Cannon Patch is a good sized patch reef and is far enough inside the main reef line
that it can be a great fairly protected place to overnight if the wind is out of the east or south. Molasses Reef and French Reef have lots of nice coral and usually lots of sea life to check out. Good anchorages are scarce on the ocean side....Rodriguez Key in Key Largo is usually good unless the wind is from the north. You can get a mooring ball in Largo Sound at Pennekamp state park, also. You can also duck into Florida Bay to find an anchorage just south of Tavernier at Snake Creek or a bit farther south at Channel 5 below lower Matecumbe. Don't try to sail Hawk Channel at night.
If you stay on the inside - there is a nice anchorage at North Nest Key on the inside just north of Key Largo. It's a couple of miles off the ICW but it's a great place to get away....during the week. On the weekend there's usually at least a few boats there. I used to go there with my Irwin 25 that drew 3 feet with no problem, and if you are careful to watch your depths you'll be OK with 4. The anchorage at Sunset Cove is nice also but exposed if the wind's from the west. From there you have access to the Publix grocery store (about a mile walk). Go to Calypsos for fish tacos and sangria if you stop at Sunset Cove. Seriously. If you miss that look for, find, and go to the Lorelei in Islamorada for good live music and coldbeer. Farther down there's a nice anchorage just west of Lignumvitae Key also, and while you're there go check out Lignumvitae Key itself. IIRC It has the last remaining stand of native mahagoany forest left in north America....it's a gorgeous sight. Don't miss Bahia Honda State Park.....cool place, interesting history, and one of the few real beaches in the Keys. After that yopu get to Marathon, and that's about the extent of my experience sailing the Keys.
Wow - didn't mean to ramble on but thinking about being in the keys got me going.
Either way - inside or out - you'll have a ball. Just watch the depths....the bottom is all sharp and pointy on the outside and deceptively shallow and grippy on the inside.