The A350 used to be called XWB, which stands for Extra Wide Body. The A350 has a larger fuselage diameter than A330. Is that a desirable feature for your airline's customers? A350 also has a higher max differential pressure, which results in a lower cabin altitude during cruise. Is that a desirable feature for your airline's customers?
A350 fuselage is also almost all composite. This has certain (promised) maintenance advantages (no corrosion & almost no fatigue issues). Is that a desirable feature for your airline's maintainers? These factors are also promised to result in the composite aircraft holding its value better after 10 years of service. Is that a desirable feature for your airline's bean counters? (assuming of course that the promise is delivered?)