In most scenarios, the rogue is better off in the melee group by a small margin, unless both are forced into a baseline situation. That being said, assuming any reasonable amount of gear and competency a ret paladin is ALWAYS better than another rogue if there is a question of having a ret paladin or not. If your guild is set up in such a way that the ret paladin or rogue would have to sit in a baseline group, you would probably get more raid benefit from dumping the rogue and recruiting another DPSER who can do more than 1645 dps after calculating raid benefit and group buffs (pretty much any dpser). Also, don't forget that a ret paladin brings in an extra set of blessings AND judgement of wisdom refreshing, which together are worth a VERY hefty amount of raid dps.
SHORT ANSWER: If you don't have a ret paladin in the raid, kick a rogue and get one.