Mainly that at base, Data Spike has a DV of your Attack plus net hits; whereas Resonance Spike is your net hits, with likely a lower limit (unless you're comfortable with resisting Fading equal to or higher than your Charisma). While Data Spike is resisted twice, (Intuition + Firewall, then Device Rating + Firewall) it will generally still score more damage due to it's dramatically increased DV.
Lemme do some maths to make sure I'm not talking out of my butt. First on someone with a Renraku Sensei (DR 3) because that seems to be a common one (looking at the NPC stat blocks in the book).
Software 6 + Resonance 6 VS Willpower 4 + 3 Firewall. Without looking at limits, that's... One or two net hits on average, with you needing at least Level 4. Assuming you have Willpower 6 (best case scenario) you've got 12 dice to resist it, so I suppose on average you're okay. Two damage an action with prettymuch no recoil.
Cybercombat 6 + Logic 6 (possibly more common than Willpower 6? Or a Logic 5 using Psyche) vs Intuition 4 + Firewall 3. Assuming you've got a Charisma of 4 (for the same limit) you've got the same number of net hits on average. However, you're also adding Attack (4) to the DV. However however, now they're rolling DR + Firewall (only 6 dice) and so get an average of two more hits. This leave you doing 4 damage a pass. You have a lower chance of hurting yourself as well; as they'd need to beat your roll, which is less likely than you rolling one less hit with your 12 Fade Resist dice.
If you've got any marks on them, the damage increases more dramatically, but then you'd have to math out "is the increase in DV enough to make up for the lost actions" along with taking into account using Brute Force or Hack on the Fly with penalties to put multiple marks on... Which I wouldn't know how to take account for.
If the TM has higher Charisma (even just one more, and if they're an elf...) then Data Spike is even more potent, and you'd have to start expecting drain damage every turn to get at least as high a limit.
Quickly though, if the target has a Transys Avalon, then they're resisting with much more dice. Assuming the rules work to allow DR used in place of attributes whenever it's higher, then Resonance Spike and Data Spike are both 12 vs 12 dice, and Data Spike begins to carry more risk of causing damage while Resonance Spike's average damage drops to almost nothing. In this scenario, getting lucky with Resonance Spike would require higher Fading values and result in one or two damage at best. Data Spike however, keeping with 4 Charisma, doesn't pull enough weight either with about the same chance of the TM damaging themselves in the attempt. At this point though, higher Charisma or putting marks on the opponent causes Data Spike to become capable of actually doing damage. Quite a bit with three marks; each one adding 2 more potential damage. If we assume that Resonance Spike is still going at Level 6 and getting pretty lucky to get 2 net hits for three straight actions, it still only breaks even with Data Spike while almost certainly causing damage to the TM every time. If the TM is instead using Hack on the Fly, they won't risk hurting themselves, and there's prettymuch nothing a ganger's commlink can do to a TM after it has marks on them since it can't be used to attack back.
