Auto constraints vs Frame Setting with 2 Buttons Example

Main.storyboard

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<document type="com.apple.InterfaceBuilder3.CocoaTouch.Storyboard.XIB" version="3.0" toolsVersion="7706" systemVersion="14E46" targetRuntime="iOS.CocoaTouch" propertyAccessControl="none" useAutolayout="YES" useTraitCollections="YES" initialViewController="vXZ-lx-hvc">
    <dependencies>
        <plugIn identifier="com.apple.InterfaceBuilder.IBCocoaTouchPlugin" version="7703"/>
        <capability name="Constraints with non-1.0 multipliers" minToolsVersion="5.1"/>
    </dependencies>
    <scenes>
        <!--View Controller-->
        <scene sceneID="ufC-wZ-h7g">
            <objects>
                <viewController id="vXZ-lx-hvc" customClass="ViewController" customModule="Centered" customModuleProvider="target" sceneMemberID="viewController">
                    <layoutGuides>
                        <viewControllerLayoutGuide type="top" id="jyV-Pf-zRb"/>
                        <viewControllerLayoutGuide type="bottom" id="2fi-mo-0CV"/>
                    </layoutGuides>
                    <view key="view" contentMode="scaleToFill" id="kh9-bI-dsS">
                        <rect key="frame" x="0.0" y="0.0" width="320" height="480"/>
                        <autoresizingMask key="autoresizingMask" flexibleMaxX="YES" flexibleMaxY="YES"/>
                        <subviews>
                            <button opaque="NO" contentMode="scaleToFill" contentHorizontalAlignment="center" contentVerticalAlignment="center" buttonType="roundedRect" lineBreakMode="middleTruncation" translatesAutoresizingMaskIntoConstraints="NO" id="DCM-PA-2n7">
                                <rect key="frame" x="0.0" y="430" width="160" height="30"/>
                                <color key="backgroundColor" red="1" green="0.76074701497824126" blue="0.077083629598191372" alpha="1" colorSpace="calibratedRGB"/>
                                <constraints>
                                    <constraint firstAttribute="height" constant="30" id="Vsa-Bj-t8M"/>
                                </constraints>
                                <state key="normal" title="Sign In">
                                    <color key="titleShadowColor" white="0.5" alpha="1" colorSpace="calibratedWhite"/>
                                </state>
                            </button>
                            <button opaque="NO" contentMode="scaleToFill" contentHorizontalAlignment="center" contentVerticalAlignment="center" buttonType="roundedRect" lineBreakMode="middleTruncation" translatesAutoresizingMaskIntoConstraints="NO" id="Nlj-mh-blN">
                                <rect key="frame" x="160" y="430" width="160" height="30"/>
                                <color key="backgroundColor" red="0.76527254342485662" green="1" blue="0.18492738676057363" alpha="1" colorSpace="calibratedRGB"/>
                                <constraints>
                                    <constraint firstAttribute="height" constant="30" id="UEX-AG-lLf"/>
                                </constraints>
                                <state key="normal" title="Register">
                                    <color key="titleShadowColor" white="0.5" alpha="1" colorSpace="calibratedWhite"/>
                                </state>
                            </button>
                        </subviews>
                        <color key="backgroundColor" white="1" alpha="1" colorSpace="custom" customColorSpace="calibratedWhite"/>
                        <constraints>
                            <constraint firstItem="2fi-mo-0CV" firstAttribute="top" secondItem="DCM-PA-2n7" secondAttribute="bottom" constant="20" id="0IM-MD-OnH"/>
                            <constraint firstItem="Nlj-mh-blN" firstAttribute="leading" secondItem="DCM-PA-2n7" secondAttribute="trailing" id="B80-dK-Z6b"/>
                            <constraint firstAttribute="width" secondItem="DCM-PA-2n7" secondAttribute="width" multiplier="2:1" id="EeP-TX-gJS"/>
                            <constraint firstItem="DCM-PA-2n7" firstAttribute="width" secondItem="Nlj-mh-blN" secondAttribute="width" id="ZWJ-3L-jng"/>
                            <constraint firstAttribute="trailing" secondItem="Nlj-mh-blN" secondAttribute="trailing" id="l9g-5s-8Ta"/>
                            <constraint firstItem="2fi-mo-0CV" firstAttribute="top" secondItem="Nlj-mh-blN" secondAttribute="bottom" constant="20" id="nl0-Xg-bLY"/>
                        </constraints>
                    </view>
                    <simulatedScreenMetrics key="simulatedDestinationMetrics"/>
                </viewController>
                <placeholder placeholderIdentifier="IBFirstResponder" id="x5A-6p-PRh" sceneMemberID="firstResponder"/>
            </objects>
            <point key="canvasLocation" x="298" y="466"/>
        </scene>
    </scenes>
</document>

ViewController.swift

import UIKit

class ViewController: UIViewController {
    
    var signupButton = UIButton()
    var registerButton = UIButton()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        configureButtons()
    }
    
    override func viewDidLayoutSubviews() {
        
        signupButton.frame = CGRectMake(0, 100, view.frame.width/2, 30)
        registerButton.frame = CGRectMake(view.frame.width/2, 100, view.frame.width/2, 30)
    }
    
    func configureButtons() {
        
        signupButton.setTitle("Sign Up", forState: .Normal)
        signupButton.backgroundColor = .grayColor()
        
        registerButton.setTitle("Register", forState: .Normal)
        registerButton.backgroundColor = .greenColor()
        
        
        view.addSubview(signupButton)
        view.addSubview(registerButton)
    }
}