Q:

Lightning SLDS Confusion

Hi I have few question on SF -SLDS:
1.  When should i use Ui:Button, and when should I use lightning:button? 
2.  I want to align slds-form-element__control with in slds-form-element to right.  How to do that? 
Code:

<div class="slds-grid slds-grid--wrap">
    <div class="slds-size--1-of-12"></div>
    <div class="slds-size--10-of-12 slds-container--medium slds-container--left">
        <br></br>
        <div class="slds-form--horizontal">
            <div class="slds-form-element">
                <label class="slds-form-element__label" for="expenseName">Enter Expense Name</label>
                <div class="slds-form-element__control">
                    <ui:inputText class="slds-input" aura:id="expenseName" value="{!v.expenseName}"/>
                </div>
            </div>
            <div class="slds-form-element">
                <label class="slds-form-element__label">Enter Expense Value</label>
                <div class="slds-form-element__control">
                    <ui:inputNumber class="slds-input" value="{!v.amount}"/>
                </div>
            </div> 
            <div class="slds-grid slds-grid-wrap">
                <div class="slds-size--3-of-4"></div>
                <div class="slds-size--1-of-4">
                     <div class="slds-form-element">                                   
                                    <div class="slds-form-element__control">
                                        <lightning:button class="slds-button slds-button--neutral" label="Create" onClick="{!c.createExpense}"/>
                                    </div>                  
                     </div>                    
                </div>
            </div>
        </div> 
    </div>     
    <div class="slds-size--1-of-12"></div>
</div>

A:

Lightning:button automatically takes care of styling in the form of SLDS so if you are trying to not custom style the CSS use that .It integrates lightning design system style buttons . 
Ui namespace type components do not provide styling so use them only if lightning namespace is not available . 
Update 
The press event is equivalent to the onclick event and should not have any impact and you can still assign an aura:id and use find syntax as you would use with ui:button
For the alignment of the button follow the below markup
<aura:application extends="force:slds">
<div class="slds-grid slds-grid--wrap">
<div class="slds-size--1-of-12"></div>
<div class="slds-size--10-of-12 slds-container--medium slds-container--left">
    <br></br>
    <div class="slds-form--horizontal">
        <div class="slds-form-element">
            <label class="slds-form-element__label" for="expenseName">Enter Expense Name</label>
            <div class="slds-form-element__control">
                <ui:inputText class="slds-input" aura:id="expenseName" />
            </div>
        </div>
        <div class="slds-form-element">
            <label class="slds-form-element__label">Enter Expense Value</label>
            <div class="slds-form-element__control">
                <ui:inputNumber class="slds-input"/>
            </div>
        </div> 
        <br/>
         <lightning:button class="slds-button slds-button--neutral slds-input__icon--right" label="Create" onClick="{!c.createExpense}"/>                 
    </div> 
</div>     
<div class="slds-size--1-of-12"></div>

Notice the class i have added to make it align right "slds-button slds-button--neutral slds-input__icon--right"

