It's tough to be sure. The Klingon Empire, certainly, has been described more thoroughly than the Romulan Star Empire has in canon, therefore there may be some cognitive bias as a result. There was, at one point, a Romulan-Klingon Alliance, which was shown to exist during the time of Star Trek: The Original Series, but which has not existed for seventy-five years prior to the time of Star Trek: The Next Generation.
The Federation Starfleet seems to regard the Romulan Star Empire as a formidable potential belligerent. This suggests a technological and military strength approaching that of the Federation, or of the combined military and technological strength of both the Federation and the Klingon Empire. Whether it is the former or the latter would depend on how the Federation views the nature of its alliance with the Klingon Empire; that is, whether it is merely a mutual non-aggression pact, or one which also extends to a defensive and offensive alliance, with each party assuming duties to defend the other. In making this determination, it should be considered that the nature of the Federation-Klingon Empire alliances have progressed from a total absence during the time of Star Trek: Enterprise and Star Trek: The Original Series, to a definite peacetime alliance during the time of Star Trek: The Next Generation, to a multi-partite membership in a wartime alliance in Star Trek: Deep Space Nine ProfessorTrek 09:49, August 11, 2011 (UTC)
By the way, in drafting the above response, I assumed you meant the Klingon nation and the Romulan nation, and not individual groups of Romulans and Klingons. We do see Klingons and Romulans living together in one episode of Star Trek: The Next Generation; but rarely, if ever, have we seen hand-to-hand combat between a Romulan and a Klingon ProfessorTrek 10:54, August 11, 2011 (UTC)
If you do mean hand to hand, there is a scene in that DS9 episode where O'Brien repetavitly travels to the future, where they are seen fighting hand to hand, and they look roughthly equal, I have little more idea, after all they both posses superhuman strength, and are both of a millitary culture.
The biggest factor (IMO) is the timeframe for the war. After the Dominion War, I think that the Romulans were in better military shape than the Klingons (the Klingons entered hostile operations much sooner, and stayed in battles for longer periods of time)... But once Romulus (and presumably Remus) was destroyed by the rogue supernova energy wave, then I think the balance would shift decidedly into the Klingons favor.