In 2014 you don't buy a computer without a SSD, unless you are on an extremely tight budget. In short, it transforms what the computer feels like. Processor speeds, etc. do not matter in day-to-day use at all, having a SSD does matter.
If your space needs are modest enough - especially since you are not on tight budget - I'd go with just SSD. For my next PC desktop, I'm going to grab a 500GB SSD at minimum (their prices just dropped to $200) but might go up to 1TB (which is about $400 but might drop to $350 by that point). Mechanical drives still have a place for cost-effectively storing large quantities of data that is needed rarely - like if you have 1TB of ripped Blu-Ray movies at 20GB each, no point in paying for SSD space for that. The important thing is that OS, apps, all files you touch often are on the SSD, so the mechanical HDD never even spins up unless you specifically go access something from that drive.
Apple charges very high prices for SSD upgrades (and it's pitiful that they don't have some quantity of SSD as default on iMacs). Their SSDs are fast compared to average consumer SSDs, so they are providing value for money in a way, but unfortunately that speed difference is not really something the average consumer will notice, so all they are left with is the sky-high price.
If I was personally buying an iMac, I'd probably either buy it with the 256GB expensive-but-fast Apple SSD, or if that's not enough space, buy it with the stock HDD, buy my own regular 500-1000GB SSD for $200-400, and pay $50 (?) for the shop to install that SSD before I take delivery of the iMac. (Of course, that option leaves you with the stock 1TB HDD in hand, which is a perfectly sized backup disk or external media storage; you just need to grab a $30 USB enclosure to put it in.)