sparing few details on how they implement, protocol behaviour on juniper and cisco is same. compared to IOS, JUNOS cli is very easy to learn and JUNOS cli provides plethora of different options to implement stuff which is not possible in IOS. Juniper have designed JUNOS which is very simple and at the same time vrey powerful so learning it would be not be a challenge. So i would strongly recmmend you to take the opertunity. Lot of big SP's have diversified network inventry and multi-vendor experiance will defnetly add lot of value to your resume in future.