Went online, found some numbers, and did some math.
Titanium has a weight of 283.39 pounds per cubic foot.
Aluminum has a weight of 168.48 pounds per cubic foot.
Titanium is 40% heavier than Aluminum
Titanium has a strength to weight ratio of 288.
Aluminum has a strength to weight ratio of 214.
Titanium has a 25% better strength to weight ratio.
Titanium has about 3 times the strength of 6061 aluminum T6. The strength to weight ratio of titanium Is only 25% better than aluminum, because aluminum is 40% lighter.
Now I was going to look up the weight of the exhaust for you too, but I figured a little math never hurts.
So find the weight of your exhaust system in one of the materials and then apply the percentages I've listed. That will give you your weight difference.