Both have advantages. If you ever plan on pulling a lot of weight (trailer) the solid axle is your best bet.
For ride comfort the IRS is the way to go.
I've owned both and enjoyed riding the IRS bike a lot more. These day's I do very little "pleasure" riding though and own a solid axle bike. The biggest reason I went with the solid axle was price and less moving parts. If I still did a lot of all day mud rides the IRS would have been a must have.
