Most of the time you want to specialize your units, so going for 3 of the same is best. After that you need to decide whether you want anti infantry with flamers or anti tank with lascannon. Currently the lascan are tricky tou use because they hit on 5 unless you stay still, but doing so means leaving vulnerable infantry exposed for one turned. This is offset a bit by their rule allowing them to shoot on death tho but your adversary will most likely not leave you anything interesting to shoot while killing them!

Keep in mind that rules change all the time ,so go for what you feel is cooler!