The two are incomparable. In terms of military might and economic power, Austria was always the nation which rose from under the boot of Napoleon, ready to have another swing. Despite losing massively and dramatically at Marengo, Ulm and Austerlitz the Austrians were able to revive and face the French equally only years later. They could match the Emperor on terms of military might, territorial size and just general
power. They were truly his only Continental equal.
But, it is is correct to say the influential role the Austrians maintained was matched by the Spanish. The Spanish, though, were just as important in terms of guerrila warfare and economic quagmire. They drained Napoleon of money and men (with at least 200 000 soldiers guarding Iberia during the Pennisular War in 1809) which could be spent resisting a Coalition offensive in both Germany and Italy, which he still won.
In fact, the two relate directly to each other. The Spanish, fake allies of the French Emperor, occupied a large percentage of his wealth and power. This allowed the Austrians more air to breath in Central Europe, and most likely meant that it delayed the disastrous battles of the wars. The guerrila warfare of Spain would not have lasted without the huge Austria, the constant threat on France's border countless times which meant that he always, always, had to re-deploy troops back to Germany.
For Mount&Musket though, this mod is primarily played in terms of line battles. This is two armies facing each other equally, a feat the Spanish could never produce during the Napoleonic Wars, though the bravery and courage of the common man was just as effective. The Austrians though, would always produce a hundred thousand strong legion in hopeless attempts to stem the French tide. That is why they should be the next nation, because in terms of military contribution they contributed the most.
Of course the Spanish did field many large armies, and once we've got Austria done (and thus the major players) I would say do Spain next, then the Ottomans/Swedes/Dutch/Portuguese/Danish...
