I suspect it'll depend on the context. Diffusion of Firewall looks good, but you're going to need a good roll to get past a decent defence pool (unless you use edge), and you are also going to be using it only against a Device or Persona, not against a Host. I do suspect though that a Technomancer who plays Decker without a Deck isn't playing to it's strengths, but I've not played one. Trying to have all the attributes and skills needed for hacking as well as Charisma, Compiling, Decompiling and Resonance will be spreading things thinly I suspect.
Running the numbers, comparing a Decker with 16 dice, against a Technomancer with 14 (12 for Resonance+Software). Say we're hacking something simple like a Commlink on an average person. So 3 Willpower, 5 Firewall on defence. Decker has Sleaze 6 with program, Technomancer Intuition 5, say. Expectation of success for Decker is 82%. Expectation of success for Technomancer is 73%. Going for 2 marks, these would be 65% and 53%. If Technomancer first used Diffusion of Firewall, at level 5, they'd have a 21% chance of reducing firewall by 1, 20% chance of reducing by 2, 23% chance of reducing by 3, 36% chance of not reducing it, and expect to lose (say Willpower 5 too, hard getting all these high attributes) 2 stun. Average will be 1 reduction (1.3 to be strict), so that's up to 79% chance of success, 60% for 2 marks. So for the cost of 2 stun the Technomancer has nearly caught up.
However, that's routine. If you were having a difficult one off device hack I'd prefer to be Technomancer I think. I'd open up with Diffusion of Firewall with edge, significantly lowering defences for all subsequent actions, whereas edge for the Decker would only affect one action. Of course, in a Host hack, give me a decker (preferably buffed by a Technomancer!)
My gut feeling is compared to other characters, Deckers don't have enough ways to improve their dice pools through advancement, and nor do Technomancers, but I could be wrong. Still, a big part of that is probably Data Trails not yet being out.
Looking through the rules, and considering Aaron's (I think) note on Noise stopping low device items getting wireless bonuses, it seems to me that Noise is designed as a way to situationally reduce augmented/decker ability, and Background Count to reduce Mage/adept ability. With that in mind, it seems to me that if a GM is throwing noise around more than background count they might be unintentionally penalising one character type (more so if not applying Aaron's note).
Edit - Ref Deckers and not needing Willpower I disagree. It's used for resisting Brute Force, Biofeedback, Jacking Out when linklocked, resisting Trace Icon and about half the IC attacks. But certainly agree on Charisma.