Both have quite good reputations, regardless of the internet posted crap.
Both use cranks and rods sourced from China, these are quite robust and haven't had any problems past the initial few years when machining was hit-or-miss. That's over now.
The 440Source kits use Icon forged pistons, a nice piston. I assume the Muscle Motors kits use an equally nice forged piston.
If you know exactly what you want, the 440Source kits are usually less expensive. The Muscle Motors kits have a big engine building shop behind them and could possibly help you with some questions more than the source.
