Both are great. The si is better for a daily driver. 

The Si is sporty but still comfortable. The Tyoe R is an all-out racer.

For daily duty, I'd go Si. For weekend toy, the Type R.

Between the two, I would lean towards the Civic Si as it would be cheaper, looks less “boy racer”, rides smoother, fast/fun enough for most people, and just a better daily driver.  That being said, it only comes with the 1.5L turbocharged I4 engine which is (or has previously been) subject to oil dilution.  The Type R has a bigger 2.0L turbo I4 which also is subject to some oil dilution (but not as bad as the 1.5L).  Not sure how long he plans to keep the vehicle as well as how many miles he intends to put on it, but long term reliability/longevity of those engines is still unknown.

For oil dilution mitigation, use the new GL-6 engine oil and change every 3,000-5,000 miles (every 3,000 to be really safe).  Also see other tips below for oil dilution mitigation:

https://carkiller.com/scottykilmer/qa/honda-1-5l-turbo-engine-with-gas-direct-injection/

https://carkiller.com/scottykilmer/postid/66706/

I would think twice about both as both are turbocharged. If he is dead set on one of the two have him drive both and pick whichever one he likes better.

I would vote for the si. 

Okay so I am a owner of both civic type r and civic si. But are good cars fast and reliable.

Type R is meant to be used on tracks not drag race 

While si is meant to be driven on road and track both

Both need premium gas and sports tires 

Si comes in both manual and automatic 

While type r is only manual 

Si has vtec in engine 

Type r has vtec in exhaust and turbo in engine 

Type r has 306 horse power 

Si has 190 to 210 horse power 

Type r is$ 40000 to 50000

Si $25000 to 30000

So your choice now 

R if money is not an issue, supposed to have 306 HP, woooooooo!

https://www.autoweek.com/car-life/a34361988/do-i-want-a-honda-civic-si-or-the-honda-civic-type-r/

Maybe I'm missing something, but I don't think they're doing an Si for '21.  Is he looking used?  Planning to buy when '22's come out?

In general, I'd recommend a Civic Si for fun daily driving (and longevity), and the type R as a weekend toy to thrash on and/or take to the track.  If he isn't going to really abuse it, I would say that the extra cost of the type R probably isn't worth it over the Si (which, by the way, has been a great car).

Both are excellent, the SI is probably more ideal for daily driving with its more comfortable suspension.