Both are great setups. REMUS and Eisenmann are both extremely reputable manufacturers that produce OEM and racing applications for a wide variety of well known teams and manufacturers. In terms of quality, you cannot go wrong with either one, it really depends on the tone and your budget. The REMUS is going to be a bit more affordable in that end.
You will need to cut off the stock muffler system, the new muffler can either be welded or clamped into place.