Marketing.
7.2K drives are slower and easier to produce, and with higher error thresholds which improves yields (and capacity). However, in terms of I/O operations each discrete disk can support, the 7.2K drives are markedly less performant than their faster brethren. Therefore they get the 'Nearline' moniker, as they'll hit I/O saturation much faster than an equivalent number of 10K or 15K disks. Therefore the storage producers need a way to convey, "we have faster stuff," so they went with MDL/Nearline.
This is how they try to encourage people who need both fast and lots of storage to go for the faster drives. Those on a budget will see that you can get (for example) a 1.5TB MDL/Nearline for half the price of a 450GB 15K drive and wonder why the upcharge. Even so, 48 7.2K RPM drives will still outperform 12 15K RPM drives. It's just that the 48 7.2K RPM drives will probably have a capacity of 30TB, where the 12 15K RPMs may only have 5TB of capacity.
Which is another way of saying...
Go 7.2K RPM when capacity is your number one goal and performance not really a goal.
Go with 15K RPM when performance is your number one goal, and capacity secondary.
The HP Smart Array P410 is a fine controller, but you will get poor performance out of it if you don't have the battery-backed or flash-backed cache units installed. The cache makes a tremendous difference in that writes are buffered by the cache memory before being committed to disk. You get the write confirmation to the application without having to incur the latency of the physical disk drives.
Here's a 4GB dd on a similarly-spec'd system (DL380 G7 with 24GB RAM and a p410 with 2 x SAS disks and 1GB Flash-Backed Write Cache). The RAM helps a lot in a test like this, but you get the idea...
[root@xxxx /]# dd if=/dev/zero of=somefile bs=1M count=4096
4096+0 records in
4096+0 records out
4294967296 bytes (4.3 GB) copied, 3.70558 seconds, 1.2 GB/s
But realistically, your write performance with two SAS drives in a RAID 1 on that controller with the appropriate cache should be between a sustained 130-170 megabytes/second. A quick iozone test on the above server configuration shows:
[root@xxxx /]# iozone -t1 -i0 -i1 -r1m -s16g
Write
Avg throughput per process = 166499.47 KB/sec
Rewrite:
Avg throughput per process = 177147.75 KB/sec
Since you're using ESXi, you can't run online firmware updates. You should download the Current Smart Update Firmware DVD, burn it to disk and make sure your system is patched to a relatively recent level.
Here are the controller's quickspecs:
http://h18004.www1.hp.com/products/quickspecs/13201_na/13201_na.html
You will want to purchase one of the following, ranging from $350-$600 US:
512MB BBWC
512MB Flash Backed Write Cache
1G Flash Backed Write Cache
To answer your question, the cache solution will help the most. Additional disks won't make much of a difference until you handle the caching situation.
*Note for other users. If you have cache memory on recent HP controllers with up-to-date firmware, there is a write cache override available if you have RAM on the controller but no battery unit. It's slightly risky, but can be an intermediate step in testing what performance would be like on the way to buying a battery or flash unit.
Best Answer
Storage performance can get a bit complex, but the underlying statement is true. 10k or 15k have similar performance, for the most part. If you put them side by side doing the type of work that disks are worst at (random small-block IO), you will measure a difference, but the reality is that with most storage controllers, that's pretty rare.
These days with most storage being able to put hot spots onto their own tier, the need for a heavy 15k tier is greatly reduced, because most of the IO read-intensive work can be put on a higher tier of SSD. In my environment, the only place we really need them is for enormous databases that stand still 99% of the time, but need screaming performance for quarterly and yearly reports that will touch almost all the data.