I found both of them to be fine. However, there is one important difference when it comes to weather conditions. Apparently, TACA (and Star Peru for that matter) is not certified to land in Cusco during difficult weather conditions. I was scheduled to fly TACA from Puerto Maldonado to Cusco in August, and the flight was cancelled (actually, it was rerouted to fly to Lima without stopping in Cusco) due to bad weather in Cusco. All LAN flights landed without problems (and they used the same planes, A319, on this route). They just rebooked me on the flight the following day, no accommodation provided; some passengers opted to travel to Cusco by bus (12 hours from PEM), but TACA refused to pay for their bus tickets and also refused to reimburse their tickets on the spot - the passengers were asked to contact their sales office. In the end I got to Cusco earlier than the bus passengers, who arrived at around 3 PM instead of 8 AM due to snow on the way.

The same thing happened when I flew TACA from Arequipa to Cusco in February: the flight was about 1 hour late as they were waiting for the weather to improve in CUZ. LAN flew there without problems. Otherwise, when the weather is good, TACA is fine: new, clean airplanes, polite cabin crew, and their fares are generally lower than on LAN.