The 2005 Magnaflow exhaust should/will fit. It is only some of the larger cannon style pipes which have had clearence issues with the rear diffuser on the 06+ models and some of which have realeased 06 revised models to fix this clearence issue.
My personal pref for exhausts is the APS 3". Quality is 2nd to none and the cat is removable piece. It is also a true 3" exhaust unlike many other brands which bottle neck between downpipe and cat back. Best of all, it is not a big obnoxious looking tip on the back, it looks close to stock, and has a very nice low tone/rumble WITHOUT drone like many other cannon styles have.
From a manufacturing perspective, I bet you would be very hard pressed to find a striking difference in materials or construction among many of the popular brands for the STi, no matter the price tag. Ultimately, it is up to your preference in sound, so that you too can come into these threads and have a subjective answer.