I travelled for 6 weeks around Europe in Nov-Dec 2008 (so autumn/winter) by train and then did 6 weeks around Europe in May-June last year (so spring/summer) by plane... I have to say spring/summer beats autumn/winter, its warmer, weather is nicer, instead of the sun setting at 4.30pm its up till 9-10pm so much more sight seeing time etc although obviously in winter its less busy, better availability of places to stay and probably lower season surcharges...
some of the pre christmas festivities were nice in some of the cities i visited though, so if you get into all that roasting chestnuts and hot drinks in the snow stuff its good... i like the warmth myself...
Travelling around on ryanair/easyjet i usually budget for around A$70 a flight, some are cheaper, others more expensive, so about 10 flights or so shouldn't be too much over A$700 if you get them early or choose the cities which are most cost effective... and yes they can get expensive if you have heavy luggage, i usually always over pack so the usual routine was to wear about two pairs of pants/jeans, 5 shirts/tops and stick as much stuff in the pockets as possible before boarding to get luggage weight down...
Trains are good, but although Europe looks small on a map it was like 14 hour train ride from Budapest to Venice, it was an overnight train but i was woken up for passport checks like every 2 hours by the Croatia police, so if you go out of the EU countries those border crossings are a bit of a PITA... i was going to do a train from Florence up to like Monaco then down to Barcelona etc but after looking at timetables it was going to be the better part of a day or more travelling... in the end i opted for like a 3 hour plane trip to Porto in Portugal for a couple of days then a 2 hour or so flight to Barcelona... those trips would have been long by train or bus... to get from Munich to Geneva took me three connecting train trips so that again took at least half the day if not more like a full day...
so again, depends if your wanting to see cities/countries located fairly close to each other or are happy to spend long stretches on trains (the scenery between Zurich and Salzburg was pretty spectacular as you climbed through the snow capped mountains)... sleeper cabins in trains aren't necessarily that cheap and many of the hostels i slept at (just looked at
www.hostels.com and the like) were often only around $15-25 a night so not always a great saving... the couchsurfers that hosted me were free and they have greater availability of places in autumn/winter as well...
In Egypt i was in a hurry and so did travel on sleeper trains to leave my limited days open for exploring... here's Eurail's website (
Eurail Pass - Prices and Info about the Eurail Passes & Tickets) they have lots of options but the more you travel and the more countries you include the more expensive it gets especially if you aren't under 26...
With planes you do open up a lot more cities to be reached in reasonable times so the width and breadth of Europe and top of North Africa if your interested, but trains/buses you do get to look at the scenery and have this non-flying backpacker mystique but not necessarily that much cheaper...

so swings and round abouts...
if you have any specifics queries to ask, or any questions about an itinerary let me know...