Statistics: Lightning vs. Cloud.

Lightning
Lightning is a fantastic physical attacker and competent attack spellcaster. What makes her interesting is that she's also really, really damn fast.
Pros: Strong, Sturdy, Good Attack Spellcaster, Very Fast
Cons: Limited Healing, Not Quite as Physically Strong as Cloud.

Cloud
Like Lightning, Cloud is also physically strong and a solid mage. Some would argue he is slightly better in both areas. However, Cloud has the distinction of being one of the slowest characters in FFVII.
Pros: Even Stronger and Sturdier than Lightning. Better spellcaster and healer.
Cons: Not particularly notable difference otherwise. Much, much slower.

Edge: Lightning. Let's be real, the comparitive differences between Cloud and Lightning aren't very big at all. However, Lightning is clearly much, much, faster. Both in-game and during FMV's. Cloud's slightly tougher, but it doesn't mean much when Lightning strikes.