Hi there,

I do understand that you want to try other teams and clients. But you will have several opportunities to do that during your career at consulting. Right now you have a team that wants you and in which performed very well… You have a highway in front of you. I cannot stress enough how much valuable that is for a fast track career. 

Moreover, you always perform better when working with teams and clients you already know (no need to spend time learning about the company and the team, etc.). It doesn't mean you have to stay forever, but taking the secondment project will help you get a 5* performance that will pay dividends later on.

This is of course your career, and only you know for how long you plan to stay. If you want to stay long and want a fast track career, you should stay with the same team and the same client. If you want to learn about the other topic or have an experience as diverse as possible, then obviously go for the other project.
 

Now let me address this specific statement, as you have it wrong: “secondment might have disadvantages for an associate as I need to network within my own company."

Yes, you have to network within your own company… when at higher levels in the company, when it becomes more political. But at your level, what you really need is a champion who cares about you and really wants you happy (i.e., promoted). At your level, networking is important to be able to get staffed… but given that you had that 2nd staffing option, it doesn't seem like you really need to do this, people in the office seem to already know that you are performing well and are trying to “grab” you into their teams.

Hope this helps, and good luck with your decision.