@Becca21 This really depends on your timeframe, the level of risk you're willing to take, and your goals (saving for a house or retirement).
Savings (cash LISA) is usually best if you know you'll want the money in a shorter time period, say less than 3/5 years. There's no risk to losing money with savings as you're given interest from the bank as your return.
Investing (S&S LISA) is usually for longer term plans, like minimum 5 years. If you invest money today and you want to buy a house next year, the market may be down over the next 12 months, which then if you withdrew the money to buy a house you're doing so at a loss. Having said that the market could go up in 12 months too but unlike savings you can't know how much return you'd get with investing. 5 years is usually mentioned as a good amount of time where the ups and downs of the market will average out where you're more likely to make some gains overall. Over a longer timeframe, these gains should be far more than the savings interest rates.
Either one you choose you're still getting 25% guaranteed bonus anyway