Upgrading memory but not storage? (MBA)

Apologies in advance for my lack of knowledge about the complexities of storage and memory...

I want to buy a 2013 Macbook Air soon. It'll be replacing my old iMac (1.83 GHz Intel Core 2 Duo with just 512MB of memory and 160GB of storage), which I bought in late 2007.

According to Disk Utility, I'm currently using only 95GB of (160GB total) storage on my iMac, whereas I've had ongoing problems for a while now with my computer being suuuper-slow when trying to run even seemingly simple programs like Microsoft Word or Safari.

So when I buy a Macbook Air, I figure I'll definitely upgrade to the 8GB RAM option. Would it be strange, then, to just buy the basic 128GB flash storage model?

I'm not necessarily planning on this Macbook Air lasting me as long as the iMac did before I upgrade to something newer (i.e. in 3-4 years, most likely), so my thinking is, if I've only used 95GB of storage in 5.5 years, wouldn't 128GB suffice now? (I'm not planning on transferring any of my current files to my new computer except for ~5GB of music). And if it turns out I do need more storage, adding an external hard drive would be pretty simple, right?

I'm just wanting to get some opinions from those more in the know about this - does upgrading the RAM to 8GB but opting for only 128GB of storage make enough logical sense?

(For more background: I mostly use my computer to browse the internet (including streaming TV shows). I'll have access to download Photoshop through my work, which I might do to get better at using it, but I wouldn't be saving a huge amount of image files long-term for that.)

Thanks for any input you can give!

 

Thanks to both of you for your input! This gives me something to chew on.