How is the short term frequency stability of a crystal oscillator?
There are several sources of frequency error in a crystal.

The first is the initial tolerance. This will be specified in the data sheet. Very cheap crystals may have been adjusted by the manufacturer to +/- 50ppm, more expensive ones to fractions of a ppm.

The next is temperature coefficient. Cheap crystals will vary by a a few ppm per degree C, expensive ovened ones by fractions of a ppm per degree.

The next is ageing. Over the course of months, and years, the frequency will slowly change. Aging is fastest when the crystal is driven at high power. Cheap ones ... you get the general idea now.

Finally we have phase noise. The frequency of a crystal will vary from uS to uS, mS to mS, and second to second, by small amounts, due to various mechanisms. Cheap ones ... and you know the rest.

If the initial tolerance difference of two crystals exceeds the peak sum of the variable terms, then the same crystal will always be faster than the other one. If it's less, then they will swap places as their frequencies vary. 

The best way is to measure your two particular crystals against each other, and plot a distribution of the frequency difference, so you can determine the standard deviation. If the mean difference is more than 10 times the standard deviation, then you're quite likely to see the same one always faster. If the mean difference is less than a few times the standard deviation, then you're likely to see them switching places for which is fastest from time to time.