Analysing fusion and fission

We analyse two extremely brave couples: Peter and Jen, aka the Siamese twins, who are a classic fusion couple, and Chloe and Yannis, who have a very free fission relationship and at times seem to lead totally independent lives.  

Our four guineau pigs all claim to be happy in their relationships, but problems lurk underneath the surface... 

Which is better: fusion or fission? Obviously a balance betweeen the two is ideal, but what are the pros and cons of fusion and fission?