is a Delta Sigma ADC more accurate than a direct conversion ADC
The number of bits usually is a good indicator for the performance of an ADC. To quantify the true performance other measures like ENOB (effective number of bits) are better.

A delta-sigma ADC with an ENOB of 16 bits is as good as any other converter with that performance. However, delta-sigma converters consist of a noise shaper and a decimation filter. The resolution is determined by the order of the noise shaper and the decimation factor. For some converters it is possible to change the decimation factor and trade off speed for resolution.

Oversampling and averaging can also be used for Nyquist rate converters, but the improvement is usually not so high, since no noise shaping is done. 

A direct conversion ADC requires \$2^{n-1}\$ comparators, so their resolution is usually lower than that of delta-sigma converters. A larger number of comparators also means better comparators (since we have a higher resolution), therefore they are much more costly.