Wallet of Satoshi is custodial, so it is not wise to keep a substantial amount of sats on it. Up to each person what “substantial” means. Some are okay with more risk than others. For solely on chain mobile wallets I prefer BlueWallet. It gives you much better control over your funds and transactions than Muun. However, BlueWallet doesn’t perform on chain to lightning swaps like Muun does. So if you send from Wallet of Satoshi to BlueWallet on chain, for instance, you will pay whatever swap fee WoS charges. There are other swap services out there too but with all of them the fees vary, which is why I’ve used Muun for that. Best approach is to learn what each wallet is useful for and develop a strategy that works for you.