@Jeff McClendon or @abenson can probably do a deeper dive, but from a purely technical standpoint I think it breaks down like this...it depends.
Not trying to be snarky, it's just that the question as you presented it is a little open ended since you didn't specify whether you were primarily interested in just general nuggets or also being able to Sniff out micro, subgram gold. Furthermore, ground conditions matter for this comparison.
I'll give you my thoughts from a purely technical basis from a detector standpoint (I own both and have done some air tests), but avid gold hunters like Steve H., Jeff, Andrew, phrunt and others who have used both can weigh in with their practical field experience.
I think overall, it's a close call based on test and real world results I've seen posted here and elsewhere. Probably a slight performance edge goes to the Nox because of the overall versatility that Simultaneous Multifrequency brings to the table in terms of hot ground handling. Maybe Orx handles itself slightly better in ferrous trash situations. I know Deus 2 does much better than the Nox 900 in that regard and also in regards to Target ID accuracy/stability. The D2 is held back slightly, however, because the smallest available coil is the 9" round.
If you ignore ground handling, the Orx may have a slight edge in subgram gold sensitivity solely based on the higher frequency (80+ khz) it can achieve with the 10x5 HF coil. I don't think the Nox 900 is far behind though, if at all, especially with the 6" coil (vs. the 10x5).
On larger nuggets I would probably give the edge to the Nox as that Coiltek coil has great depth for its footprint on larger gold targets vs. the Orx. It's still close.
For hot ground handling, the Nox in Simultaneous Frequency Mode is better than the SF Orx and also has ground tracking unlike the Orx. But the Orx still performs ok in hot ground and is easy to rebalance with single button ground grab.
Finally, the Nox 900 is an overall more versatile detector vs the Orx for detecting in general.
So overall edge to the Nox. Orx will cost you a lot less however ($550) vs. $1000 for the Nox (includes the 11" and 6" coil) + $235 for that Coiltek 10×5.
TBH however, based on the recent $500 price drop on Manticore down to $1200, I would also strongly consider getting Manticore over the 900 and pick up the $250 M8 5x8 coil which should give you everything you need (it effectively combines the best of the 6 inch round coil and the 10x5 coil with perhaps a incremental hit on depth and coverage). The cost Delta is only about $200 even if you factor in the cost of the 10x5 Nox coil vs. M8 Manticore coil But you do end up with one less coil overall because the Nox package includes the 11" and 6" coils. Also, the Manticore is just a better all around detector than the 900. There is nothing the 900 can do that the Manticore can't. FWIW.
My recommendation:
For pure nugget hunting value, consider the Orx.
For overall performance Manticore + the M8 coil.
Splitting the cost difference (slightly, by $200) Nox 900 + Coiltek 10x5.
@Jeff McClendon or @abenson can probably do a deeper dive, but from a purely technical standpoint I think it breaks down like this...it depends. Not trying to be snarky, it's just that the question as you presented it is a little open ended since you didn't specify whether you were primarily interested in just general nuggets or also being able to Sniff out micro, subgram gold. Furthermore, ground conditions matter for this comparison. I'll give you my thoughts from a purely technical basis from a detector standpoint (I own both and have done some air tests), but avid gold hunters like Steve H., Jeff, Andrew, phrunt and others who have used both can weigh in with their practical field experience. I think overall, it's a close call based on test and real world results I've seen posted here and elsewhere. Probably a slight performance edge goes to the Nox because of the overall versatility that Simultaneous Multifrequency brings to the table in terms of hot ground handling. Maybe Orx handles itself slightly better in ferrous trash situations. I know Deus 2 does much better than the Nox 900 in that regard and also in regards to Target ID accuracy/stability. The D2 is held back slightly, however, because the smallest available coil is the 9" round. If you ignore ground handling, the Orx may have a slight edge in subgram gold sensitivity solely based on the higher frequency (80+ khz) it can achieve with the 10x5 HF coil. I don't think the Nox 900 is far behind though, if at all, especially with the 6" coil (vs. the 10x5). On larger nuggets I would probably give the edge to the Nox as that Coiltek coil has great depth for its footprint on larger gold targets vs. the Orx. It's still close. For hot ground handling, the Nox in Simultaneous Frequency Mode is better than the SF Orx and also has ground tracking unlike the Orx. But the Orx still performs ok in hot ground and is easy to rebalance with single button ground grab. Finally, the Nox 900 is an overall more versatile detector vs the Orx for detecting in general. So overall edge to the Nox. Orx will cost you a lot less however ($550) vs. $1000 for the Nox (includes the 11" and 6" coil) + $235 for that Coiltek 10×5. TBH however, based on the recent $500 price drop on Manticore down to $1200, I would also strongly consider getting Manticore over the 900 and pick up the $250 M8 5x8 coil which should give you everything you need (it effectively combines the best of the 6 inch round coil and the 10x5 coil with perhaps a incremental hit on depth and coverage). The cost Delta is only about $200 even if you factor in the cost of the 10x5 Nox coil vs. M8 Manticore coil But you do end up with one less coil overall because the Nox package includes the 11" and 6" coils. Also, the Manticore is just a better all around detector than the 900. There is nothing the 900 can do that the Manticore can't. FWIW. My recommendation: For pure nugget hunting value, consider the Orx. For overall performance Manticore + the M8 coil. Splitting the cost difference (slightly, by $200) Nox 900 + Coiltek 10x5.