Excellent, this is always a good subject with many many opinions. Actually there is a rule that works for 95%+ of cases, hosting should be 1% of revenue to be in the top 5% (which is where half the revenue is generated), the top-down view (the percentage decreases the less you want to be at the top). If you choose based on hosting price first your revenue will be reflected on a bottom-up approach, but most of the time will still match the 1%. There are various reasons for this but Google and Visitors are very sensitive to your hosting which controls ranking and checkout performance.
SSD is a nice to have one you have non-cached speeds ideally 2-3s or less, this is the same principle for FPC, as the file system should be rarely hit if you set it up correctly, otherwise the benefits are maginal (not that you will be told this as hosting providers love upselling SSD). Magento is CPU intensive so the answer all depends on caching you have (memcache, apc, varnish), web server software (apache, litespeed, nginx), number of products, numbers of websites/stores, extensions, custom code, theme using ajax.
The 100sales per day is pretty funny, under normal business circumstances you are then generating $1.5million revenue per year, on the cusp of EE (from $2million to $10million), and should be running on clusters with 100,000 visitors per month to generate the 2-3% conversion rate to get to 3,000orders.
If you are running on a shared hosting you have no choice of the technology, but if a vps or dedicated then you need to choose your whole architecture. SSD is a technical choice to squeeze some simple performance out (20% improvement) without doing anything, the RAID will perform better if your caching architecture is correct. Only you can decide which is best.