It is interchangeable.
My core nuance is that:
- an sich reißen = you gain power over sth./ you possess sth. now
- etwas unter den Nagel reißen = you own sth. now
Thus, the first might come with the latter: if you gain power over an object (de jure: possession (Besitz)), it might also be yours (de jure: property/ownership (Eigentum)) [now].
As already known: In both cases it was not yours and - at least for bystanders - it looks like the one changing of ownership/ possession took the other(s) by surprise/ cought them unaware of the situation/ opportunities. If it is or feels unfair depends on circumstances and point of view.
That means, that if anybody else would have had the same thoughts and overview of the situation, they would "simultanously run for the thing to gather". Just some realize it to late/ don't have capacities to do (like to less money to buy, less uncaring to take advantage).
My distinction in usage:
- an sich reißen: I use that more for taking the power that comes with a job/function/authority role
- unter den Nagel reißen: I use that more for taking objects
DWDS has examples for "an sich reißen" - and most refer to positions/ jobs:
- Kommando an sich gerissen (~ took command)
- die Präsidentschaft an sich riß (~ took presidency)
In wiktionary are examples for "unter den Nagel reißen" - and all refer to objects, not positions/ jobs:
- die teure Uhr unter den Nagel reißen (~ being the first one to take watch)
- Anbauflächen unter den Nagel reißen (~ being the first buyer for the agriculture area)