They're both great companies that can offer a lot of bang for the buck. Whichever you choose make sure you stick with that company; don't mix and match parts.
There's nothing wrong with mixing and matching parts if you know what you're doing. My AI car hs a hodge-podge of parts from different manufacturers, and some custom made stuff I got cheap:
K-member: Griggs (Lighter weight & more anti-dive compared to MM.)
A-Arms: Maixmum motorsports (stronger than Grigg's tubular A arms.)
Bumpsteer: Off-the-shelf rod ends, MM tie rod sleeves, MM tapered steering arm adaptors.
Torque Arm: Custom
Panhard Bar: Custom
LCAs: Griggs (Lighter weight, and adjustable spring perch.)
The combo works very well, but I know the repercusstions of each part and what it needed to do and not do.
Find someone local to you that can offer you a ride and decide from that.
Unless the cars are setup similarly and are running the same tires on the same track, you're not going to get a very useful comparison.
In general, Grigg's suspension parts are more race-oriented and that means lighter (meaning, unfortunately, that they will be more likely to break, over time.) Griggs stresses that its parts are race parts and need periodic inspection and maintenance.
Maximum Motorsports parts are MUCH stronger, and unfortunately, heavier.
I built my AI car for racing, thus it has Griggs lightweight parts on it, particularly the K-member. If I were building a street car or a weekend track toy, it'd have Maximum Motorsports parts on it.
Both companies make excellent parts and support them very well. The improvements you'll see with either system are HUGE. You can't go wrong picking either company, IMHO.