Short answer
A blazer is a standalone jacket you pair with contrasting trousers. A suit jacket comes with matching trousers from the same cloth. The blazer is more versatile for mixing; the suit jacket is more formal as a set.
The blazer: your wildcard
Navy, cream, velvet or tweed — designed to be worn with different trousers. The most versatile jacket you can own.
The suit jacket: your formal anchor
Cut from the same cloth as the trousers, designed to be worn as a set.
Which should you buy first?
If you wear suits to work: the suit. If you need a jacket for smart-casual: start with a blazer.
Questions people actually ask
Can I wear a suit jacket as a blazer?+
Technically yes, but people notice when the jacket has no matching trousers.
Is made-to-measure available for blazers?+
Yes — at Bonturberg you can commission a standalone made-to-measure blazer.