<button class="btn btn-primary {!v.isLink1 ? 'active' : ''}">
<button class="btn btn-primary {!v.isLink2 ? 'active' : ''}">

Field Integrity Exception: Failed to save undefined: Cannot mix expression and literal string in attribute value, try rewriting like {!'foo' + v.bar}: Source

<button class="{!v.isLink1 ? 'btn btn-primary active' : 'btn btn-primary'}">