AR-15
Pros:
Accurate.
Reliable if properly maintained.
Great iron sights.
Cheaper mags than a Mini..
Parts are widely available.
Much better looking than a Mini-14 (IMO).
Available in far more configurations, and can be changed from an "urban carbine" into a long-range varmint or match rifle by simply switching uppers (of course, a new upper will cost you the price of a Mini-14 or more)
Cons:
Fairly expensive ($700 and up).
Filthy action, very high maintanance (must be cleaned after EVERY shooting session).
An evil black baby-killing assault rifle
<there - so nobody else gets confused>(not necessarily a con, it certainly doesn't bother me, but it is banned in some of the more tyrannical areas, such as NYC and Kalifonia).
Mini-14
Pros:
Very reliable.
Inexpensive.
Lower maintanance.
Politically Correct (again, not a pro in my eyes by any means, but it is legal in places where AR's are not).
Cons:
Not very accurate.
Ugly (IMO).
Expensive mags (when you factor in 5 or so 30 rounders, the price difference between an AR and a mini becomes less of an issue).
Parts not as widely available, and Ruger will not sell them to you, you'll have to send the rifle to their shop if it is broken (their shop is very good from what I have heard, however).
Not very versatile -- a Mini-14 is what it is, a reliable, but not very accurate and kind of clunky rifle. It will never be a long range varmint rifle or a quick handling "urban carbine".
I'd say definately go for the AR if they are legal where you live and you don't mind working a little to keep it clean. The price difference shrinks when magazines are factored in.
I have outshot quite a few people with scoped Mini's at the range with my Bushmaster Shorty AK with a 14.5" barrel and iron sights.
Have fun.
[This message has been edited by Andrew (edited February 18, 2000).]