Just from a short comparison (under full moon conditions) evaluation of the AT 102 ED.
Lasted 20 minutes after twilight ended and before the clouds rolled in.
The scope is a winner !
fantastic focuser - smooth inside/outside focus with no image shift
APM 30mm UFF was perfectly flat to the edge - 23.8x FOV 2.73° Surprisingly dark background considering the full moon interference. Double Cluster was perfectly framed - beautiful pinpoint stars.
used the same eyepiece and located NGC 7789 with averted vision with just a hint of a few stragglers
M13 was nicely resolved with an 11mm DeLite and easily took a 5mm DeLite (143X) with snap focus
Next clear night the shootout vs the Orion 120mm f/5.
Methinks the metric here for comparing these two scopes (e.g. 120 f/5 Achromat vs. 102 f/7, um, "APO") will be when (read: at what magnification?) does chromatic aberration become apparent? Well, at least that's the metric I'd be using.
Comparing to either either Sedwick or Conrady's CA criteria as found here...
CA-ratio-chart-achro.jpg (JPEG Image, 797 × 426 pixels)
...the 120 f/5 will exhibit an "Unacceptable Levels of CA" ratio of 1.06.
Now, were the 102 f/7 a mere Achromat it would already find itself in the "Filterable Levels of CA" zone with an approximately ~1.7 ratio!
However, the 102 being an "APO", I should think, will push its CA ratio much-closer-to or into the, um, green "Minimal or No CA" zone. (Perhaps someone here knows or can provide the mathematics for determining an accurate CA ratio number for APOs?)
The comparison I'd be most interested in finding out at the eyepiece would be, "at which magnification did each instrument begin to reveal objectionable levels of CA?".
My guess here (based solely on my own observational experience over the years with a smaller 102mm f/5 Achro) is that the larger Crown + Flint 120mm will best best suited for rich-field viewing at magnifications less than 0.5x its aperture (in mm)...120mm x 0.5 = 60x-and-under before CA bespoils the view.
The wildcard here will be how well your FPL-51 + unknown-mate 102ED is actually corrected for CA. Based on my reading user comments here on CN regarding that model (and its clones) it would appear that (depending on a user's sensitivity to CA) the scope should be able to provide relatively CA-free views up to around 1.5x its aperture (in mm)...102mm x 1.5 = ~153x-and-under, making it, in my book, both a rich-field scope and a decent planet-viewing scope.
As an RFT the 120 f/5, I should think, will prove preferable for its brighter views. And, as a planet scope, I would hope to be able to pull out the 102 f/7 for some .7mm exit -pupil viewing.
Anyhoo, I'll be curious to read, and look forward to, your comparisons and assessments.
Then, of course, there is the question of image contrast when comparing the two scopes!