Online is easier these days and I thought quicker processing times too (was when I extended earlier in the year) though according to the processing times as of now paper applications are currently well ahead with processing times of 24 days as opposed to 50 days for online applications:
http://www.cic.gc.ca/english/information/times/temp.asp
Up to you, I suppose if you think you might be turned down then go for whatever is taking longer to process which would give you a few more weeks on "implied status". I think though you can submit an online application right up to and including the day your status expires (least you can for Visitor Records, might be different for visas not sure there's tools on the mycic site that takes you through all that) and still maintain a legal status, not sure if it's the same for paper.
