First of all, Chili (MTML network) does not seem to have any "fast" mobile internet offer. Its website mentions GPRS or EDGE as technologies, some kind of receivers having a speed of up to 3Mbps. It seems confirmed by its absence from the list of LTE networks.
Regarding the two other networks, Orange/Mauritius Telecom and Emtel, they both have started deploying a 4G-LTE network on the island in 2012. There does not seem to have any map of the coverage for either network, but some press articles give an idea of the deployment of the network.
In terms of speed, Orange announced up to 100 Mbps and Emtel up to 150 Mbps. They both use the frequency of 18000 MHz.
In terms of coverage, an article stated that both covered at the end of 2013 Port-Louis, Ébène and more communities, both expanding their network, and suggested that Orange had a wider network, targeting a full coverage of around 50 communities in 2015. Orange claimed in 2012 that its 3G network covered 75% of the island territory.
Also note that Mauritius Telecom/Orange also has an operating network on the island of Rodrigues. It is 3G+ but anyway it goes through a satellite connection to Mauritius with a speed of 155 Mbps (which I expect is for the whole island, so do not expect too much from that).