I've been a Celtic and Man Utd fan since I was a kid (since they won the European Cup one year after the other in 67 and 68), but I back up my credibility by saying that I'm from Saltcoats, where Bobby Lennox lives, and that I have lived in both Glasgow and Manchester, unlike many Celtic/Man U fans worldwide.
Man U weren't always the world beaters they are now, by the way, they have been relegated in the past (73-74). Tommy Doc took them down and brought them back up.
Celtic, on the other hand, have always been easy to support. Not since the glory days of Ferguson and McLean with Aberdeen and Dundee Utd have the Old Firm had any consistent and meaningful competition in the SPL.
So when I lived in Irvine, in sympathy with my mate John, I started supporting Kilmarnock. I've probably only been to Rugby Park six or seven times in 20 years (my first being Charlie Nicholas's debut for Celtic) but I still check out their results weekly.
Kilmarnock are a much, MUCH better team to support. They offer the Scottish football fan enough of the misery needed to nourish a Presbyterian psyche, with just enough success to stave off the pangs of despair.
For those who don't follow the Scottish game, Kilmarnock also have an Aussie playing for them, Danny Invincibile (yes, Invincibile) who's from where I live right now, namely Brisbane.
If Kilmarnock start getting too successful, I'm definitely looking at Gretna.