**TL;DR:** Both methods are about the same, because most of the energy is needed to overcome the ["enthalpy of vaporization:"](https://en.wikipedia.org/wiki/Enthalpy_of_vaporization) the last push required to go from 100degC to boiling. 

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### Basic formula
From [an answer on Chemistry.SE](http://chemistry.stackexchange.com/a/27312/37235), this is the formula for energy needed to boil a liquid:

`Q = (mass)*(specific heat)*(change in temperature) + (mass)*(enthalpy of vaporization)`

For water:

 * specific heat = 1 kcal/(kg degC)
 * enthalpy of vaporization = 540 kcal/kg

### Total energy needed
Applying the formula:

`Q_heating = (5kg)*(1 kcal/kg/degC)*(100degC - 8degC) = 460kcal
 Q_boiling = (5kg)*(540kcal/kg) = 2700kcal
 Q_total = 460kcal + 2700kcal = 3160kcal`

### Energy to preheat

From this result it's obvious that regardless of starting temperature, most of the energy is needed to overcome the enthalpy of vaporization, which can only be done by the stove. 

But all the same, let's assume I have perfectly insulated plumbing and see if the water heater can help a bit:

`Q_preheat = (5kg)*(1 kcal/kg/degC)*(49degC - 8degC) = 205kcal`

Thus the remaining energy the stovetop must add is:

`Q_remaining = 3160 - 205 = 2955kcal`

### Stovetop vs water heater efficiency

A gas stove-top is about **44% efficient** ([source](http://blog.us.schott.com/boiling-point-putting-induction-electric-and-gas-to-the-test/)), and a modern natural gas water heater with a tank is at best **67% efficient** ([source (pdf)](https://www.energystar.gov/ia/partners/prod_development/new_specs/downloads/water_heaters/WaterHeaterAnalysis_Final.pdf)).

### Final comparison

Using only the stovetop:

`Q_stovetop = 3160kcal / 44% = 7182kcal`

Preheating with the water heater:

`Q_preheat = 205kcal / 67% = 306kcal
 Q_remaining = 2955kcal / 44% = 6716kcal
 Q_combined = 306 + 6716 = 7022kcal`

So in the end, I've only saved about 160 kcal (186 watt-hours), or **2% of the total energy needed**. Given @Fred's point about the possibility of leaching from the water heater, it's probably not worth it.