Practically speaking, there's a huge difference.

RCS/iMessage are great. They're a huge upgrade over SMS, however, the E2E statements they make aren't really verifiable to the degree necessary to call them secure. They also require hardware compatibility, software compatibility, environment compatibility (root breaks RCS) as well as network compatibility so the pool of devices that work both ways with RCS is still pretty small. It's frankly a mess. Default settings for most RCS/iMessage applications will attempt to send via E2E protocols and if it fails, it defaults back to sending SMS. So now your super secret content was just sent basically over cleartext if the protocol send fails. lol

Realistically speaking, he's right. There's no difference. People don't casually message information which is important enough to require perfect forward secrecy. So at the end of the day choose which works best for you and if you do dumb shit like sending credit card and social security numbers over clearnet, then prepare to have your anus widened.

I personally prefer running an MTProto proxy on top of Telegram. I control the proxy, so I can view where the network traffic is going in transit for the most part. Is MTProto perfect? No. But it's vastly improved since previous independent audits and it's "good enough."

If critically sensitive information has to touch a device with internet access then you need a mature security protocol like PGP or some other shared key cryptography so you can verifiably ensure you're talking to whom you're supposed to be talking to. If that's something you're interested in, give Keybase a try. It's a really great platform built around a really great technology (PGP). The mobile application comes with a chat option that uses your PGP key to symmetrically encrypt your chat messages using Scrypt (with PBKDF2) making it significantly more secure than any other option mentioned here.