Arguably

[JPablo]

Oxford bilingual gives,
arguably adv (indep) this is arguably his best novel = podría decirse que ésta es su mejor novela; arguably, they would have done better to refuse = posiblemente hubiera sido mejor que se negaran

Random House Unabridged gives

arguable 1. susceptible to debate, challenge, or doubt; questionable: Whether this is the best plan of action or not is arguable.
2. susceptible to being supported by convincing or persuasive argument: Admirers agree that it is arguable he is the finest pianist of his generation.

Cambridge Klett gives
arguably = posiblemente
arguable = discutible

I.e., we would not go into translating "arguably" as "*discutiblemente".

I take this is a matter of usage, but is there a basic difference of meaning between "arguable" and "arguably" and/or in terms of usage?

(Arguably, I am splitting hairs here, but any points on any arguable difference in nuances and/or usages will be appreciated.)

[pjt33]

"Inarguable" significa "cierto" - no se puede discutir.
"Arguable" sí que es "discutible" - se puede discutir. La preposición que sigue suele dejar claro si el hablante está a favor o en contra.
"Arguably" modifica un adjetivo y suele indicar que el hablante arguye en favor de aplicar el adjetivo.

Ejemplos del BNC:
It is arguable that more recently retired business men would have a constructive role to play. (Y puesto que el hablante es un político, apuesto a que arguye que sí lo tendrían. Muchos de los ejemplos de "arguable that" son de abogados, y en eses casos podría ser sólo la incertidumbre que les interese).
It is arguable whether it would reduce the overall cost of regulation for small audit firms. (Aquí arguye en contra).
Since the last century the Tay has been known not only as the largest, but also arguably as the best salmon river in Britain. (Es probable que el hablante esté en favor de llamarlo el mejor río para pescar salmón).

[JPablo]

Thanks a lot, Pjt33!

[aleCcowaN]

¡¡Excelente explicación, pjt33!! Me aclaró mucho.

Yo diría, basado en la explicación:

"it is arguable whether ..." = "es discutible que + (subjuntivo que apunta a la inexistencia)"

"it is arguable that ..." = "puede decirse que + (indicativo que apunta hacia la existencia con eventual condicional para manejar lo hipotético y lo probable)"