I use the two-line version without braces (the 2nd form), but not to save space.

I use that form because I find it more readable, more visually appealing, and easier to type.  I only use that form if those conditions are met; i.e. the `if` condition has to fit nicely on a single line, and the corresponding statement has to fit nicely on the following line.  If that is not the case, then I will use braces to improve readability.

If I use this form, I make sure that there is an empty line (or a line containing only a brace) before and after the `if` statement (or above the comment, if present).  While this is not a rule that I consciously follow, I notice it now after reading this question.

Conserving screen space is not a priority for me.  If I needed more space I would use a larger monitor.  My screen is already large enough for me to read anything that I might need to focus my attention on.  It's unlikely that I would need to focus on so many lines of code at one time that they take up my entire screen.  If there is so much nesting going on with a chunk of code that I can't understand it without viewing more of it at one time, then I would have to consider whether the logic could be better represented by refactoring.

Below are some examples that demonstrate how I use this form of the `if` statement.

        string GuardConditions(Plan planForWorldDomination)
        {
            if (planForWorldDomination == null)
                throw new ArgumentNullException("planForWorldDomination");

            if (!planForWorldDomination.IsComplete())
                return "Doh!";

            planForWorldDomination.Execute();
        }

        void ProcessingLogic()
        {
            OneBlankLineAbove();

            if (simpleCondition)
                simpleStatement();

            OneBlankLineBelow();
            OneBlankLineAbove();

            // optional comment on the line above an if statement
            if (simpleCondition)
                simpleStatement();

            OneBlankLineBelow();
        }

        void Assignment(string drive)
        {
            OneBlankLineAbove();

            string prompt;
            if (simpleCondition)
                prompt = "simple assignment";
            else
                prompt = null;

            OneBlankLineBelow();
        }

        string Return()
        {
            OneBlankLineAbove();

            if (simpleCondition)
                return "simple return";
            else
                return null;

            OneBlankLineBelow();
        }