Soft Cups and Diva Cups are made of different materials – the diva cup is more rigid than the soft cup. They are also different shapes so one may suit you better than the other in terms of fit. Some people have used soft cups and found they leak but used diva cups and found they didn’t and vice versa.
Also with regard to using soft cups for TTC, it is better to use the soft cup rather than the diva cup because they sit in a slightly different position and are therefore able to hold in the sperm better. The thing is really you should give either or both of them a try if you are using them for your period and see which one fits your body best.