Which is better for mulching?

The SENIX. It lists mulching and side discharge, while the Troy-Bilt listing only describes a rotary blade system.

Which has the stronger deck?

The SENIX specifies a 14-gauge steel deck. Troy-Bilt does not publish deck gauge for this model, so the SENIX is the safer bet for rough yards.

Which is easier to start?

Both use a pull cord. Troy-Bilt owners specifically report first-pull starts, so it has the better track record, though the SENIX has slightly more displacement.

Is the SENIX worth stepping up to?

If you want mulching, side discharge and a published cutting-height range, yes. If you just want a basic cut-and-go mower, the Troy-Bilt covers it.

Do either of these bag clippings?

No. Neither lists a bagging option, so plan on mulching or side discharge.

Which is better for a small, flat lawn?

Either works. The Troy-Bilt is the simpler choice; the SENIX adds mulching if you want to return clippings to the lawn.

SENIX LSPG-M14 21-Inch Gas Mower vs Troy-Bilt TB120 Gas Push Mower

The SENIX LSPG-M14 is the more complete mower on paper: steel deck, mulching and side discharge, and a 144cc engine. The Troy-Bilt TB120 is a simpler 21-inch push mower with a 140cc engine and a strong record for easy assembly and first-pull starts. Both are basic gas push mowers with no self-propel and no bagging, so pick on deck material and discharge options rather than brand.

Choose the SENIX LSPG-M14 if you want a heavier steel deck and the option to mulch or side discharge clippings.
Choose the Troy-Bilt if you want a proven, simple push mower that owners say starts easily and assembles fast.

Prices

SENIX LSPG-M14 21-Inch Gas Mower: $225.97 at Amazon (checked today)
Troy-Bilt TB120 Gas Push Mower: $289.99 at eBay (checked today)
Difference: the SENIX LSPG-M14 21-Inch Gas Mower is $64.02 cheaper than the Troy-Bilt TB120 Gas Push Mower today