Buying new server - stock 512MB or 1GB raid cache upgrade?

I'm buying a new server to replace two or three old Server 2003 R2 boxes and the system I'm looking at is the Lenovo (IBM) Express x3500 M4 - in particular the 7383ENM model.

This includes: 6-core Xeon E5-2620v2, 8GB ECC DDR3, 2x 300GB 10k SAS, ServeRaid M5110, optical drive, 2x 750W PSU. I will be adding 3 more 8GB ECC modules (to make 32GB in total), 6 more 300GB 10k SAS drives (to make 8 in total) and 2x 500GB SATA6 drives.

The server will run VMware vSphere ESXi 5.5 and consist of 2-3 Windows OS guests.

If you look at the IBM redbooks page for that M5110 hardware raid controller (81Y4481), you will see there is a 1GB Cache upgrade (81Y4559) and I can buy that for around $350 extra. Both the standard M5110 and the upgraded M5110 have a battery backup, so the only difference is the cache.

After some research and various forum reads, I will probably run ESXi from a USB flash drive, but as far running the following:
one 900GB array of 6x SAS drives in RAID10 with a Windows Server 2012r2 Standard VM as an AD Domain Controller with Folder Redirection for a 35-user office
one 300GB SAS RIAD1 array for another Windows Server 2012r2 Standard VM for some small MS SQL Express applications)
and the SATA drives in RAID1 for additional minor VM's or backups

Is there any performance and reliability benefits of double the write-back cache in this particular scenario?

If so, what are they and are they worth the added cost when that may be put to better use in adding more RAM or HDDs for data redundancy? I understand that 'are they worth it' can be subjective, but I'd really appreciate any advice on the build.
ReeceICT ConsultantAsked:
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Tony GiangrecoCommented:
Raid cache can increase your drive's writing performance, but there are some items to consider.

1. The raid controller should have a battery backup on the card.
2. The server should be protected with a good UPS.

Here is a great article with pros, cons and recommendations.

Hope this helps!
ReeceICT ConsultantAuthor Commented:
Thanks TG-TIS.  Your comment is a bit vague in reference to my question(s) and the article doesn't help.
I mentioned that both the 512 and the upgrade 1024 have a built-in BBM (BBU), so the difference is only cache.

Is 512 more megabytes of cache on the raid controller really going to give me much of a write performance advantage?  Is that advantage worth ~$350 or is that better spent on say:- another 16GB RAM or more HDD's for data redundancy?
What would the recommended cache size or 'best-practice' be for a system that runs 10 or so SAS hard drives?
andyalderSaggar maker's framemakerCommented:
You won't see much difference in performance with RAID 1/10, write cache's only really useful with RAID 5/6. You would see a performance boost if you used SSD read caching "cachecade but the license is pretty expensive (and you would also have to buy as SSD).

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