Hard to say. Depends really on when the battle is fought. 1700-1709, the Swedish army was probably the best trained in Europe. After 1709, the Swedish infantry and cavalry declined sharply in quality due to large casualties in Livland and at Poltava - even so, the quality of the infrastructure producing more men was fairly good, although it needed time to produce troops of the same quality as the pre-war ones.
After 1709, Cronstedt reformed the Swedish artillery. Already advanced for its time, the regimental guns became better organised, lighter, more manouvrable and had a rate of fire 8 times that of a musket, and could fire 30-32 musket balls, something that contributed to the victory against Denmark and Saxony at Gadebusch 9th of December 1712 (at a time when Sweden should be on the ropes, fighting Poland, Saxony, Prussia, Bremen, Hannover, Great Britain, Denmark-Norway and Russia at the same time).
The Swedish army of the time was almost incapable of defence, it was completely organised around the offence, a big weakness in some instances, a great advantage in others. The Swedish basic unit, the battalion, had 2/3 musketeers and 1/3 pikemen, something abolished in the English/British army of the era. This gave the Swedes an advantage in close combat and against cavalry, but a disadvantage in shoot-outs.
The Swedish tactic was to fire two salvoes at 20 and 10 paces and then charge. Discipline was very good and unit cohesion even better. Routs at a large scale was almost unheard of during the time. The infantry had the ability to keep formation and silence under heavy enemy fire as they aproached, something that often unnerved their musket-armed opponents (Saxons, Danes and Prussians, as well as mercenaries in service of the Poles and Russians).
However, if the Swedish infantry bounced from their charge, they were in a dire situation as retreat under fire, formation changes etc were not drilled at all (IIRC).
The Swedish cavalry was also very offensive, aiming to cut-in at the enemy cavalry and drive them from the battlefield. They rode in a tight V formation knee behind knee with rapiers extended in front of them to break enemy caracolling cavalry, something they did VERY well. Pistols were for extended melees, not for caracolling, something which was expressively forbidden. They rode sturdy, hardy and small horses used to the cold climate and with a stamina for long pursuits. Driving the enemy cavalry away and keeping them away, then returning to help the infantry crush the enemy main battleline was the objective.
The situation is hard to determine. Marlborough was very smart, if he has been fighting the Swedes for a few years, or been with someone who has, he can probably design a deep defence like the Russians did at Poltava and let the Swedes come to him and win by superior fire, but if he has not, I suspect he will want room to manouvre and line up in the normal European tactic of the time, eating massive Swedish grape shot and getting charged hard.
So, if it is the first time they meet, Karl XII wins, if Marlborough has fought the Swedes before, or at least studied their tactics very well, he wins.