Ignoring all the other distinctions between lisps, the main difference between lisp-1 and lisp-2 (or lisp-n) is going to be how clean your code looks when you lean into the FP style. In a lisp-2 you'll need to do something like this:

  (defun apply-twice (f x)
    (funcall f (funcall f x)))

  (apply-twice #'1+ 2)

Versus this with a lisp-1:

  (define (apply-twice f x)
    (f (f x))

  (apply-twice 1+ 2) ;; assuming 1+ is defined

But there are so many other differences between the lisps in the two categories that this probably won't be the deciding factor for most people.

personal taste, for the most part. I like the thought of a single namespace, it fits my intuition from pretty much every other language out there, and I like how the code looks when I can pass functions around as though their names are regular variable bindings to an underlying function object.

The argument is: if you don’t have hygienic macros, a Lisp-2 is going to be less brittle than a Lisp-1.

The classic example is, imagine you have a function with a local variable called “list”, common enough. Now imagine you invoke a macro inside that function which generates a call to the built-in “list” function - also common enough. In a Lisp-1 without hygiene that breaks - your local definition shadowed the built-in; in a Lisp-2 or hygienic Lisp-1 you’re in the clear.

Having a separate namespace for functions is silly in that it only saves you from a small set of variable shadowing problems. It’s a hack, not a serious solution.

As someone who has used both kinds over many, many years: it really doesn't matter.

I am a strong proponent of Lisp 1, primarily because the distinction between functions and other types of values is artificial. Functions have first class semantics in Lisp 1 and Lisp 2, but Lisp 2 makes you denote them differently but in an inconsistent manner.

Lisp 2 advocates typically make a few arguments. One is that having a separate namespace for functions makes it clearer when you are using a function vs another value. The second is that the evaluator has less work to do when examining the head of a list - it needs only look in the function environment, not the full environment.

On the first subject I must disagree - you can bind a function to a regular variable and then use that variable everywhere (except in the car of a list representing a function call), so for most positions in a set of expressions you don't really get information about whether the object being denoted is a function or not.

I suppose the second point is somewhat valid, though I suspect if you benchmarked interpreters and compilers it would barely matter. As a person who favors functional programming with a lot of combinators, I find Lisp 2 introduces a lot of pointless noise in the syntax for no reason. And I fundamentally just don't see functions as significantly different sorts of values, so I find the syntactic distinction bizarre.

IMO if we look at Lisps today the question looks more like: SBCL, Chez Scheme, Racket or Clojure.

Common Lisp and Racket are Lisp-2s but honestly, the namespace thing seems like a minor difference compared to all the other features that differentiate them.