On this forum you'll see a lot of discussion of mods, for power, for mpg, for sound, for looks. It's possible to spend a lot of time and money on your Sonic, and get good results. I'm not writing this as a put down on anyone doing these things and FSM knows I'm as guilty of it as any.
But the truth is 90% of these mods are a waste of time and money if you don't really care deeply about and deeply enjoy tweaking your car.
Take power mods for instance. It's possible to take your 138hp car and get 250hp out of it, maybe more. But the only time you get that power is when pushing really hard, hammer down, accelerating like crazy or pushing the very top speed. If, like most people, you never go full throttle, then you aren't going to use the full 138hp much less the extra 112 you could have with a full set of mods.
I accelerate normally onto a turnpike using 25% throttle, with a stock car to get the exact same acceleration you might use 40%, but it would feel the same.
Lets look at mpg modding. Stock your Sonic can get 40mpg or more, depending entirely on how you drive it. A mod that gets you a whopping 2 extra mpg might cost $500. Over a five year car ownership span running 100K miles assuming gas costs $4 per gallon you will spend maybe $10k on gas at 40mpg, and $9556 at 42mpg, only $444 savings, a net loss.
The stock Sonic is the best car in its segment; handles great, plenty of power, great mpg. There's nothing wrong with getting one and leaving it as is.
If you want to dip a toe in the water, a couple changes really stand out;
A Trifecta tune will get you more power and better mpg while changing nothing physically about the car. The budget tune is cheap, easy and all you need unless you dive in deep. If you can find someone near you who will loan you the red cable you can save even more. If you decide to upgrade later, the budget tune price comes off the total for the full tune, so no money lost.
Replacing the Hankook tires will get you better performance; the stock tires are arguably the worst feature of the Sonic, poor grip and poor handling, you will be safer in better tires. (note, here we should discuss the best cheap tire replacement options, I got something relatively expensive and unnecessary)
Finally the engine question. The 1.4 turbo is the better engine. Better gas mileage when you don't push the car, more power when you need it. All things being equal this is the one to get. But it costs more money; $700 more on the LT and LTZ trims, worse if all you wanted was the LS trim you will have to move up to LT which means an extra $1000 on top of that $700. May not be worth it; there's nothing wrong with the 1.8.