Triggers After Insert and After Update - Good Practice
From a performance point of view if the are doing the same thing then there will be no difference. If your trigger performs tests to see what needs to be done (i.e. needs to treat updated rows differently to newly inserted ones) then there could be an impact if query plans that are sub-optimal for common cases get cached and reused for those cases.

For a code maintenance PoV the decision is also split by what differences there are in the behaviours. If the action taken in each case is identical (or near identical) then it makes sense to have one piece of code than two (so if you fix a bug in one you don;t have to remember to replicate the fix in the other location) - in this case using one trigger for both actions is preferred unless there is a significant performance difference. If the actions of each trigger for the trigger are noticeably different then keep them separate, again for code clarity reasons.

Of course it also depends what database system you are using: different ones will have different performance caveats that you may need to be concerned about.