Three of our employees started using eCenter to enter their time about six months ago. It has been wonderful and saved me a lot of typing. Just let your employee(s) know not to "Complete" a time entry until it is truly complete because once you hit the eCenter transfer button, as Administrator, all time marked as Complete will be moved. I hope this helps. I have not heard of or used iSlips so I am not familiar with that.

A few key differences between iSlips and eCenter.

eCenter is a Sage product. iSlips is written and supported by a third party.

eCenter is accessed by login in to a website. iSlips is an installed "app". It can be installed on Windows or Mac computers and on android and iOS devices. 

eCenter completed items are transferred to Timeslips when the administrator request. Once downloaded they are not visible in eCenter. iSlips downloads entries automatically into Timeslips when the synchronization app is running. The synchronization is installed on the server or a workstation and can be left running most of the time. Time entries in iSlips are generally visible to the user and can be edited (based on permissions) until they have been billed. 

In eCenter you can only enter time for the specific eCenter user. iSlips will default to the specific licensed user but you can enter slips for others.

eCenter requires you to be on Timeslips Premium. iSlips is compatible with older version of Timeslips as well as Timeslips Premium. (Not necessarily recommending unsupported versions of Timeslips, just saying it is possible).

As you can see, they have some differences. We have clients using both. Would be happy to speak with you in more detail or help with getting either on setup.