Most of the big companies stay with one brand, because they can get big discount when they buy servers. Also, if you are buying many servers, the will provide you dedicated support staffs. Easier to update the patches, bios, etc....
Definitely go with one brand. Much easier to keep spare parts around if you are going to have a lot of servers.
We have been using SuperMicro for the past 11 years and have been very happy with them. Our customer's use all kinds of servers. We've seen the most problems with Dell servers and those are the hardest to work on.
I definitely think it would be easier to keep with one type. It will help to have them the same in situation you need to restore them; discovering areas to fix them would be easier if the areas will be the same whenever you need them and create purchasing them easier.
I'm dealing with the brand Dell since last 10+ years and received awesome support and offers so I suggest you to stick with only one brand. Best of luck
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