@Lars L Can someone tell me something about the performance and the scaleability of a ASP.NET-App?
As mentioned before, I have no experience with Xojo Web, but I can say that ASP .NET performs and scales amazingly, if you know what you're doing.
@Julian S It basically boils down to concurrent connections vs time to implement vs cost.
As a small contribution to what could reasonably called a subjective metric on ASP .NET, I work with a company with offices in 70+ countries, with about 18,000+ employees.
One of my projects, which I've been working on for a few years, is a rather large internal ERP system built on ASP .NET (with a SQL Server backend).
There are 4 load-balanced IIS Webservers running ASP .NET 4.5 (with dynamic & static file compression enabled) running in VM's with the database as a two-node Cluster (one active, one passive). The servers are hosted in the UK on a private WAN.
We have on average about 8,000 active users a day, with an average of just over 500 concurrent users at any one time. Which adds up to over half-a-million application hits a day (not counting static files, i.e., images, css, js, etc.).
The database cluster is at about 30 to 40% utilization, but the Webservers , running ASP .NET are only at 5 or 10% utilization (with the occasional spike of course, because we also share these servers with other processes). Even though we don't need them right now, we could drop-in extra servers to scale up further pretty effortlessly.
The load-times are in milliseconds, or just low-digit seconds. Which is pretty good considering some of our key database tables hold millions of rows of data.
On the rare event of unexpected down-time, it's usually a network fault.
It's taken 4 years to get to this point, with the system still growing, with just half-a-dozen of us developers to manage it. But at one point, we had over 30 devs dedicated to it, as a start up cost.
With the next iteration of ASP .NET moving to .NET Core, where we'll be able to deploy to Linux Servers (including SQL Server), we expect ASP .NET to keep performing and scaling up quite nicely.
So, in my not-so-humble opinion, given competent developers, ASP .NET performs and scales exceedingly well. But it does take effort and cost.
As a further bit of information, I find the decision process for competitive project bids like this, mostly come down to what the buyer is willing to invest for a long-term return (if they're thinking long-term) and the people they trust the most - to make the technology choices for them.
In other words, don't get hung up on the technology. Either have the confidence and commitment to deliver what you know to be the right path forward - and let that shine through to your customer, or acknowledge that you may not win every bid and be okay about it.
I hope that helps.