Historically Paris-Nice has been the better race, but last year and especially this year the list of riders in Tirreno is impressive with many teams bringing their top riders. The route has got something for everyone, from sprinters to attackers, climbers and all rounders/GC men.
Obviously being an italian race all the italian teams are bringing their big guns, but also foreign teams like BMC , Astana, Katusha are all bringing pretty much their A-team and Sky, Garmin, Lotto and Greenedge are all bringing their top sprinter.
Check out the full list of riders for Tirreno (http://www.gazzetta.it/Speciali/TirrenoAdriatico/2011/it/download/TA-Iscritti2012.pdf
) and you'll see what I'm talking about.
Basically all I'm saying is that there are all the conditions for an epic week of racing.
Maybe because I don't really like the guys in contention for this year P-N (Wiggo, Leipheimer, Valv-piti), I'm a bit bias, but I think I'll be following more Tirreno this week.