There are pros and cons to using lambda functions as your AppsSync resolvers (although note you'll still need to invoke your lambdas from VTLs):

Pros

• Easier to write and maintain
• More powerful for marshalling and validating requests and responses
• Common functionality can be more DRY than possible with VTLs (macros are not supported)
• More flexible debugging and logging
• Easier to test
• Better tooling and linting available
• If you need to support long integers in your DynamoDB table (DynamoDB number types do support long, but AppSync resolvers only support 32-bit integers. You can get around this if you use a lambda, for example by serializing longs to a string before transport through the AppSync resolver layer) - See (currently) open Feature Request: https://github.com/aws/aws-appsync-community/issues/21

Cons

• Extra latency for every invocation
• Cold starts = even more latency (although this can usually be minimised by keeping your lambdas warm if this is a problem for your use case)
• Extra cost
• Extra resources for each lambda, eating up the fixed 200 limit

If you're doing a simple vanilla DynamoDB operation it's worth giving VTLs a go. The docs from AWS are pretty good for this: https://docs.aws.amazon.com/appsync/latest/devguide/resolver-mapping-template-reference-dynamodb.html

If you're doing anything mildly complex, such as marshalling fields, looping, or generally hacky non-DRY code, then lambdas are definitely worth considering for the speed of writing and maintaining your code provided you're comfortable with the extra latency and cost.