My 1983 300TD had been starting poorly when cold last fall, right before I put it in winter storage, so I ordered some new Beru glow plugs over the winter and installed them today. The old ones included three Beru gps and two Bosch; both the Bosch had failed, the three Beru were good. If remember correctly, my supplier ran out of Beru and supplied the mixed order of Bosch and Beru the last time I changed them.
I tested them using a length of 10 gauge stranded copper wire with large alligator-type clamps at each end and a motorcycle battery I had lying around; one clamp on the negative battery terminal, one on the threaded body of the gp, and touch the gp terminal to the positive post on the battery.
The engine started right up with the new gps--problem solved. I think I will stick with Beru from now on.

Glow Plug reaming discussed and shows mine as well as other home made
glow plug reamer - pic - PeachParts Mercedes ShopForum

Hunter speaking of Glow Plugs dieing due to Carbon insulation
Injectors and Glow Plugs for Dummies - PeachParts Mercedes ShopForum

Bosch Glow Plugs are good, more available and cost less; Beru Glow Plugs are better but cost more and are not as avilable.

Besides the above Carbon Build up issues the condition of your Injectors also has an effect on how long your Glow Plugs last.

Some People also have said not to mix brands within the same set of Glow PLugs. But, there is no hard evidence as to why or why not.

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84 300D, 82 Volvo 244Gl Diesel

It took me a long time to learn that lesson with a 7.3 Ford. The OEM Ford parts are Beru's.