As long as you are using a hot wallet, an iPhone or Samsung will not protect you enough from risks. It doesn't matter that much. The greater part of your security will depend on how you operate the device and what exactly you do with it.
Cold wallets are more secure and safer than hot wallets.
If you store your fund in only one wallet, it needs to be a cold wallet but remember that it's not recommended to store all your coins in one wallet.
If you store your fund in a hot wallet, that as being said, not too secure and unsafe, it is recommended to store your small fund in that hot wallet.
It is your duty to protect your phone. Also, you should look towards getting a hardware/cold wallet if you will be saving your bitcoins for a very long time.
With mobile devices and hot wallets, there are many risks to lose your devices, your wallets and your coins. Be careful is good but not enough to completely secure your coins stored in hot wallets.
Especially if it is a hot wallet but with single signature. You can try with a multisig wallet.
Wallet basics. Do not use wallets that don't give you recovery data; these wallets are likely controlling your keys.
Do not use paper wallets unless you're an advanced user who understands all the risks.
Do not store large amounts of value in single signature wallets.
Make sure your heirs know how to recover your wallets without you!