Tobago is two different worlds in terms of diving.
The populated South end near the airport (Pigeon Point) and main population center, where there are many dive ops and terrestrial attractions- having been a cruise ship port for many years- well known by many for Buccoo Reef. You can easily find partying until all hours- if that's what you might like. The Eastern shore near the airport (Crown Point) is getting built up by luxury hotels.
The North end (Speyside) which is quite rustic by any comparison (largely travelled by international "birders" with binoculars!) where more adventurous and advanced divers go for the thrill ride currents. This areas features the strongest lateral and upwelling/downwelling currents of anywhere in the Caribbean. Coz, noting your previous experience, is a nice walk in the park by comparison. In terms of partying, it is practically comatose.
Cozumel is way cheaper in every respect, including airfare certainly. You get what you pay for~ Tobago is one of those very special places that still exists in the dive world, even on an International standard. The diving to the South has more critters and corals, and the Carnival atmosphere lingers all year long- without the annoyance of having to go to Trinidad. The North is Mr Toads Wild Ride in terms of currents, much more stark, but it is truly a beautiful vision.
The island is worth touring around, one of the few places I recommend renting a vehicle. They are right hand drive and although the roads are well paved and maintained- there are no lights or reflectors- it gets pretty dark at night or when raining. The island is rich with fresh water lakes and waterfalls.
The central zone (Scarborough, Roxbury, etc) of the island is also often visited by vacationers, but I would not recommend the locations as a base of dive operation- but the accommodations can be quite a bargain.
