I'm not sure if it is right to compare the 2. Loopring is L2 blockchain for building non-custodial AMMs, orderbook exchanges and payment applications on Ethereum. Whereas, Polygon is a L2 blockchain for building and connecting Ethereum-compatible blockchain networks. So the 2 have different use cases even though both of these are L2 technologies. Please correct me if my understand is not correct.
Coming to your question of Loopring as an investment, given that loopring has a very narrow use case when compared to Polygon, I would not prefer to invest with the same expectation as Polygon.