RAID 1 on drives from different manufacturers - is it justified?

Question

LVM

Hello. There is a NAS on Ubuntu Server 14.04, a soft RAID 1 on mdadm + LVM is used as a disk system. Two hard drives of the same series (?), but with different capacities - 500Gb and 750Gb, respectively, the effective capacity of the array is now 500Gb. The hard drive models are ST9750420AS and ST9500420AS. Now the NAS is working in test mode, data has not yet been transferred to it. I checked fault tolerance by emulating the failure of one of the disks, followed by reconfiguration of the array. And everything seems to be working, but it is possible to replace the 500-ku with another disk, with a capacity of 750Gb, model Hitachi Travelstar Z5K1000 HTS541075A7E630. Confusing is the fact that this drive has a different spindle speed - 5400rpm, and the buffer size is 32Mb, unlike my Seagate, which has 16Mb. At the moment, Plus, I see replacements in increasing the effective disk space up to 750Gb, but the stability of the array, built on different disks with different characteristics, from different manufacturers, remains questionable. In principle, for my purposes, an increase of 250 marketing Gb is not particularly significant, so now I decide what to do - leave Seagate in a pair or change 500 Gb to Hitachi 750Gb?