1. Telegram is very open that they are not end-to-end encrypted by default and never portray themselves as anything else. Secret chats are e2e and nothing more.
1. Cloud chats are encrypted in transit and in storage. Encryption keys are broken up into pieces and stored in various jurisdictions, making it virtually impossible to legally force giving up data to governments.
1. Whatsapp is never secure. As there have been countless exploits in it where you can gain full access to the remote device. No such exploit, or really any, has ever existed in Telegram.
1. TG accepts 3rd party clients to it's open API.
1. TG let's you validate that the mobile client you install on your phone is the same as the source code published in their public repos
1. The backend is closed source but I always thought that was a dumb thing to mention because you have no idea what's actually running on the servers in the end.
1. Signal has suspect funding (read Surveillance Valley)
1. Signal does not allow 3rd party clients to use it's open API (suspect!) and also no way to verify your clients
1. Afaik, no government has ever been successful in forcing TG to give up any data.
1. There is a still unclaimed 6 figure bounty for anyone that can break their encryption (for years now)
1. Finally (I could go all day) I think they are the most open about whatever is going on. That comes off as genuine to me.

Yes, obviously I do like to use Telegram but I wouldn't use it, or any similar service, to send anything that was truly sensitive. Also, does appear to collect more metadata than I'd like but it's still fairly minimal.

Just my $0.02