The bottleneck of your system is HIGHLY specific to the application of your Mac. Here are some examples:
A video game rig is almost always bottled necked by the graphics
card, as video games don't require much disk access or CPU computing.
Casual use, like loading applications and files, is likely
bottlenecked by file access speed.
Development and A/V production requires a mix of most computer specifications, but there isn't an "automatic" application to determine it for you.
Here is how I would determine the bottleneck:
- Take a look at your RAM usage. A nice free way to do so is opening Activity Monitor, right clicking the dock icon, and selecting "Show Memory Usage" under the "Dock Icon" sub-menu. If your RAM (excluding "inactive" memory) is frequently high (for example, 80%), then perhaps it'd be worth upgrading. If you only ever used half of your 8GB of RAM, adding on 8 more GB wouldn't impact performance.
- Similarly, keep an eye on your disk activity (from activity monitor). Benchmark your Mac's HDD (so you learn it's Read/Write max speeds) and compare the disk activity you observe to the maximum capability of the drive. If you frequently hit the max, then perhaps it's SSD upgrade time.
On a more subjective note, I know quite a few tricks for optimizing RAM performance, which I could share with you over e-mail or something (to avoid spamming the comments here). I would recommend the SSD upgrade. Apart from the obvious file I/O speed boost, if you use too much memory, your SSD would make for much better swap space than your current HDD.