Enanthate is a stable ester that requires injections only every 5-7 days without a drop in blood levels. Technically that is not entirely correct but basic enough for your question.
As for sustanon it is a blend of 4 different esters with different release times. If you don't inject at least eod you will have blood level fluctuations which lead to a list of issues, but nothing serious. Lots of guys love sust, I hate it unless you can inject it daily or eod but that is too much of a dose for almost everyone on here.
If you want to use a fast ester to start your cycle with, just use enan and prop for the first 4 weeks of the cycle. That is closer to sust as far as putting on gains quickly but you will have more stable blood levels in the end.
The reason so many guys love and swear by sust is the quick ester release so gains are much faster than waiting the 4 weeks or so for the levels of enan to accumulate enough for gains.