First, I think the advice given you by others is right--since we are well in advance of your travel dates, you ought to get the deal on SAS from FRA to ARN--while ARN is 20+ miles north of central Stockholm, there is plenty of reasonably priced public transportation options (and you might find a decent cheap place in Uppsala, the famous college town which is only about 10 miles north of ARN). Skavsta is 5 miles or so from the small city of Nykoping, from where you could catch either a bus or a train to central Stockholm, 60 miles northeast.

Ryanair does have its place, however. I have taken 5 separate trips on Ryanair, and have had good experiences on each. You really have to play ball by their rules and follow those rules to the letter. In other words, you need to show up at the airport 2 hours in advance, to make sure that you reach the check-in counter and get your boarding pass at least 40 minutes prior to scheduled departure, or you will lose your reservation and your pre-paid airfare--no refunds, no waivers, no favors. You also need to bring your own food and drink on board if you want to eat while you fly (buy bottled drinks in the secure area of the airport), because their prices for drinks and snacks on board are very high (I recall an 8 oz. can of Coke and a small can of Pringles went for E5 in 2003). Ryanair's flights are typically short (I went to the 2006 Winter Olympics on them, STN-TRN-STN, and according to the captain the flight was 576 airline miles, the same distance as BWI-ATL here in the states, which we covered in 85 minutes), so the seating is not that big a deal, certainly not worth paying a premium for. Ryanair does have scheduled bus services to its remote airfields (for example, catch the bus to "Paris-Beauvais" outside the convention center at the Porte Maillot Metro station--the fare was something like E13 one way, and the bus reached BVA airport 75 minutes later); some of their airports, like Stansted, Prestwick and Turin, actually have regular train service to the centers of their cities; some other of their small airports, like Biarritz-Anglet-Bayonne in southwest France, are actually within a couple of miles of the center of town.

Ryanair's airports serving certain cities are ridiculous in terms of the distance from town--Hahn, 124 km (77 miles) from Frankfurt; Beauvais, 80 km (50 miles) from Paris; Torp, 70 miles from Oslo; Skavsta, 60 miles from Stockholm; Charleroi, 30 miles from Brussels; Girona, 60 miles from Barcelona; Eindhoven, 50 miles from Amsterdam; Weeze, 30 miles from Dusseldorf; Salzburg, 1 hour by train from Munich; Stansted, 35 miles from central London (although less than 20 miles from Cambridge); Prestwick, 30 miles from central Glasgow; Bergamo-Orio al Serio, 30 miles from Milan (although the major airport in the region, Malpensa, is on the other side of Milan and even further away); Bratislava, over 30 miles and an international boundary away from Vienna. A number of their airports serving secondary markets are just outside the towns they serve--Derry in the occupied six counties is a real good example of that, around 4 miles outside town.

Ryanair seemed dependable and was flying new 737-800s which it bought directly from the Boeing assembly line, so it has that going for it.