Which is better for an iPhone?

The Powertek. It uses Qi2 magnets for alignment and charges up to 15W, while the onn pad tops out at 7.5W on iPhone and needs careful placement.

Will the Powertek charge my Samsung or Pixel?

It can charge a Qi-enabled Android phone on the pad, but you get no magnetic hold and no fast Qi2 speed. The onn pad is the simpler fit for Android.

Both are plastic pads with no moving parts, so wear comes from the cable and adapter. The Powertek adds a lifetime replacement warranty, which the onn does not advertise.

Do I need a special case for the Powertek?

Yes for a firm magnetic grip. Use a MagSafe-compatible or Qi2 case; thick or metal cases weaken the hold and can slow charging.

Is the Powertek worth stepping up to over the onn pad?

If you carry an iPhone, Apple Watch and earbuds, yes: one station replaces three chargers and cables. If you charge one phone, the onn pad does the job with less to set up.

Can I travel with either?

The Powertek folds flat and fits a bag, but it is a three-device station. The onn pad is smaller and lighter for a single phone on the road.

onn 10W Qi Wireless Charging Pad vs Powertek 3-in-1 Qi2 Tri-Fold Wireless Charger

The Powertek 3-in-1 is the better buy for Apple users who want one station for phone, watch and earbuds, with Qi2 magnets and up to 15W. The onn pad is a simple single-device charger that works with any Qi phone, including Android, and its 4.1 rating from 617 buyers is a broader track record than the Powertek's 4.9 from 35. Build and features favor the Powertek; simplicity and cross-brand support favor the onn.

Choose the onn 10W Qi Wireless Charging Pad if you have one phone to charge, maybe an Android, and want a no-fuss pad that works with any Qi device.
Choose the Powertek 3-in-1 Qi2 15W Tri-Fold Charge Pad if you own an iPhone, Apple Watch and wireless earbuds and want to charge all three on one folding stand.

Prices

onn 10W Qi Wireless Charging Pad: $12.00 at eBay (last seen Nov 9)