You are interested in the relative error and/or stability, which is unaffected by any division. So, other things being equal, take the lowest ppm clock source for the best accuracy.
If 50 ppm is enough for you, I would choose the crystal for board space and price.
For better accuracy you could consider a better quality (tuned) crystal oscillator. Same physical format as a regular oscillator (14-pin DIP size), but higher price (~ $20).
My seat-of-the-pants understanding for load capacitors (corrections invited) goes like this:
When a crystal is cut for a certain load capacitance, it is measured with that capacitance across it during final factory trimming. There is nothing magical about the value. It is simply a way of saying, that if you design your circuit to present that same capacitance, then your crystal will be within the stated (.005% or whatever) tolerance.
So, you add up all the capacitance in your circuit, and then add in what's needed to bring it up to the spec. We'll use your numbers. The stray capacitance due to the traces on the board obviously will vary with the board, so let's guess 1.3 pf. A number I made up, to go with the capacitance of the microprocessor's oscillator, stated to be 1.7 pf. So, we've got 3 pf in parallel with the crystal. The crystal wants 18pf, so we have to make up the 15 pf difference with discrete parts.
Since the two load capacitors are in series (Gnd->cap->xtal->cap->Gnd), we double the cap value to 30pf. Two 30 pf caps in series give us the 15 pf we're looking for.
Note 1. I tried searching for typical PCB stray capacitance. It was all over the map. Suffice it to say, that as the hardware gets smaller, the capacitance will keep getting smaller. A lot of typical values claimed less than 1 pf.
Note 2. If there is more capacitance than spec, the crystal will oscillate at a lower frequency than specified. If there's less, then it's higher. You can see, that if you want to trim the oscillator to spec, it's easier to shoot for a lower capacitance and add some later, than to try the opposite.
Note 3. For fun, look up "gimmick capacitor".
Note 4. My "seat of the pants" explanation is sufficient as an introduction, and this technique works in many cases, but not everywhere. For a more in-depth look at the EE principles behind those capacitors, see this answer.
Best Answer
What "stability" means in this context is a bound on the effects of temperature. For example, the manufacturer may guarantee that the frequency will not change more than 10ppm over the range of -10 to +60°C. It is distinct from initial tolerance.
ESR has an effect on the pullability of the crystal and the power dissipation and probably jitter.
If you have too much power dissipation you can cause the crystal to drift over time just from that cause.
Personally I would be inclined to pick the one with lower ESR unless precise timing was of some great importance in the application. Or maybe the one with high availability, pedigreed supplier and lower price.