mjhjmtu

What's the difference between :

It's 2018 already and I still use an iPhone 5S.

vs

It's 2018 already and I am still using an iPhone 5S.

Does the use of the word 'using' mean I am thinking of upgrading really soon, or is there any other significant difference?

With the "It's 2018" clause, both mean more or less the same thing (as Neil says). But without that clause to clarify, the implication would be quite different.

I still use this phone would be something you say to emphasise the fact that generally-speaking, you still use the phone. If someone suggested you throw the phone away, for instance, you could say "I still use it", or if you wanted (as in your original example) to emphasise that despite it being old (or perhaps broken in some way) you still use it.

I'm still using this phone, on the other hand, more emphasises the fact that you are currently in the act of using the phone — not just that you use it from time to time. For instance, if someone has lent you their phone, and they ask for it back, you could say "I'm still using it" to indicate that you haven't yet finished with it, but you'll probably finish shortly.

One is simple present tense while the other is present continuous.

Use simple present whenever you want to indicate something you do frequently.

Every friday, I go to the mall.

Use present continuous whenever you want to indicate something you do in this very moment.

See ya later, I am going to the mall.

In this case, you could use either, frankly. They both seem to imply that you're ready for an upgrade, but neither moreso than the other.

As you may know, the simple present tense is used for natural, repeating, or habitual activities. The present continuous is used for current and ongoing activities, things that are in progress.

In this case, do you habitually use your phone every day? Or are you using it continuously? It's kind of the same thing, don't you think? It's just two slightly different perspectives of the same general activity.

So when talking about your 6-year old phone, it doesn't really matter which you say. Nor does it imply any special intention to replace the phone. Both are just statements of fact.

I expect this is the same with most other activities in a similar context, but you should be aware of the nuance in case it does make a difference: