You could have both? Why not have a risky upgrade, with a chance to destroy item/lose overall upgrade slots/fail and lose chunk of change/etc, this could provide additional bonuses, maybe an item proc or special effect for 5/10 or 10/10.
Then if people aren't game for it, a guaranteed success upgrade which costs more/same/less but gives less stats?
Or you could have 5 for risky upgrade and 5 for safe upgrade? Something along those lines.
The min-maxers always want better stats, and this introduces gold sinks/item sinks if/when trading comes around.
Personally, I don't like taking risks quite like that, so the alternative guaranteed but weaker upgrades would probably be my thing, but it would at least allow the choice.