Hello,
That's not a simple question. Roatan is an island. Belize is a country with many diving destinations (the atolls, Ambergris Caye, Placencia, etc.).
In terms of travel, you can fly directly into Roatan from the US on Continental or Delta. The airfare is often quite high because of the low competition and relatively low demand. Also, the flights only run on certain days of the week. You may get a lower fare if you fly into San Pedro Sula and then take a one or two hop puddle jumper to Roatan but that adds time. You can fly into Belize everyday but since there is only one international airport in Belize, you must take a second hop to your final spot.
In terms of diving, that depends. I've been to Roatan numerous times and to Ambergris Caye twice. I have not visited the other parts of Belize (including the atolls). If one is only comparing Roatan with only Ambergris Caye, I personally prefer Roatan. The walls on Roatan are beautiful. Ambergris Caye has lots of "spur and groove" areas. It comes down to preference.
With regard to non-diving activities, I agree with the above poster. Belize has so much to offer in terms of Mayan ruins, hiking, etc. The tourism infrastructure is pretty well developed. As far as conveniences, with Ambergris Caye in particular, there are a lot of restaurants, bars, etc. that are all in town and very easy to access. You don't need to take taxis or rent cars. San Pedro is quite walkable. Roatan is more spread out. Depending on where you are going, taxis can be very expensive. The other parts of Belize may be a different matter altogether.
Belize is an English-speaking country. While some residents who have come from neighboring countries may not speak English, English is the dominant language. Roatan has an English history. While the native islanders often prefer English, you will encounter more of the Spanish language since that is the national language of Honduras. You will find more native Hondurans who do not speak English. With the tourism areas (hotels and dive shops) you will have no problem speaking English. The "flavor" of the locations is somewhat different.
So...as far as which is better...that kind of depends. Sorry to not give a simple answer, but your question is a little more broad than you might realize.