(Async & await) vs (without async & await) in web API

I am new with Async and await using C# Programming. In WebAPI, we have created two API Controllers one with Async and await Programming and other is without that. We have done load testing using JMeter and we have got following results.

    Users          Sync                                         Async
________________________________________________________________________________
    100           No Errors                                    No Errors                             
    500           No Errors                                    No Errors                             
    750           No Errors                          Errors - (59.0 %) - 502 Bad Gateway   
    763           No Errors                                    Errors                                
    764           No Errors                                    Errors                                
    765           Errors - (0.13 %) - 502 Bad Gateway          Errors
    1000          Errors                                      Errors                                


Can you any please explain/suggest which approach is best or how can we proceed ?

What I have tried:

API Code :
====================================================================================
GetPersonalDetailsController - Async and await Used
====================================================================================

    public async Task<IHttpActionResult> GET([FromUri] RequestQueryListDTO objAPIRequest)
    {
    	DateTime startResponseTime = DateTime.Now;
    	Response objResponse = null;
    	string strResponse = string.Empty;
    	var HeaderType = Request.Content.Headers.ContentType;
    	ProductBAL objProductBAL = null;
    	try
    	{
    		if (objAPIRequest != null)
    		{
    			Task<Response> tskGetProductDetails = Task<Response>.Run(() =>
    			{
    				objProductBAL = new ProductBAL();
    				return objProductBAL.GetProductDetails(objAPIRequest);
    
    				//Business Access Layer Logic calling
    			});
    			objResponse = await tskGetProductDetails;
    		}
    		else
    		{
    			objResponse = new Response();
    			objResponse.ReturnCode = -1;
    			objResponse.ReturnMessage = "Missing Parameters.";
    		}
    	}
    	catch (Exception ex)
    	{
    		\\ Exception Logging
    	}
    	finally
    	{
    		objProductBAL = null;
    	}
    	objResponse.ResponseTime = Math.Round((DateTime.Now - startResponseTime).TotalMilliseconds).ToString();
    	if (objResponse.ReturnCode == Convert.ToInt32(General.ReturnCode))
    	{
    		return Content<Response>(HttpStatusCode.BadRequest, objResponse);
    	}
    	else
    	{
    		return Ok(objResponse);
    	}
    }
		
		
========================================================================
GetPDPController  - Without using Async and await
========================================================================

    public IHttpActionResult GET([FromUri] RequestQueryListDTO objAPIRequest)
    {
    	DateTime startResponseTime = DateTime.Now;
    	Response objResponse = null;
    	string strResponse = string.Empty;
    	var HeaderType = Request.Content.Headers.ContentType;
    
    	try
    	{
    		if (objAPIRequest != null)
    		{
    			//Business Access Layer Logic calling
    		}
    		else
    		{
    			objResponse = new Response();
    			objResponse.ReturnCode = -1;
    			objResponse.ReturnMessage = "Missing Parameters.";
    		}
    	}
    	catch (Exception ex)
    	{
    		// Exception Logging Code
    	}
    	finally
    	{
    		objProductBAL = null;
    	}
    	objResponse.ResponseTime = Math.Round((DateTime.Now - startResponseTime).TotalMilliseconds).ToString();
    	if (objResponse.ReturnCode == Convert.ToInt32(General.ReturnCode))
    	{
    		return Content<Response>(HttpStatusCode.BadRequest, objResponse);
    	}
    	else
    	{
    		return Ok(objResponse);
    	}
    }