In Swords & Wizardry Core Rules, the Cleric will eventually become a better fighter than the Fighter, once you get into the higher levels. If two people in the same party play together all the way through, and get the same experience in every adventure, at 920,000 xp, the Cleric will hit level 20 while the Fighter is still at level 15.
The Cleric will have more hit points, and will be +11 to hit while the Fighter will only be at +10 to hit. So, in a straight-up fight, the Cleric will have a slight edge on his Fighter buddy. And to top that off, the Cleric gets spells and Turn Undead.
Of course, the Fighter player will likely have better stats in Strength and Constitution, so this will be mitigated a bit. But a Cleric player who is looking to be the front-line guy will compensate for that with putting one of his best scores in Strength. So, if you plan on playing a long-term campaign, there’s no good reason to have any Fighters in the party, at least with the published rules.
A number of people have posted variant fighters, and I fully support this. In my campaign, I think I will be increasing the rate of to-hit bonuses for Fighters (maybe a flat +1 per level), and maybe give them a combat style to choose from, as suggested in Knockspell magazine. That will make them a worthy option for an adventuring party. But without tweaking, I don’t think the Fighter class is comparable to the Cleric.
Of course, if you are only going to play a short campaign, this really won't affect your game at all. But I think in the long term, this problem should be addressed in the main ruleset.