This depends on the situation. If the transmission has minor issues, such as worn-out seals, it may be more cost-effective to simply have it rebuilt. However, if the transmission has major internal damage, such as a cracked housing or damaged gears, it may be more practical to replace the transmission entirely. Additionally, some cars have transmissions that are more expensive to rebuild, making replacement a better option if cost is a concern. Ultimately, the decision to rebuild or replace the transmission will depend on the specific circumstances and the cost of parts and labor involved.