In an enterprise network with (2800,2900,ISR,ASR routers) what is better for DmVPN or FLEX-VPN?

It's global question, many things will be effect in enterprise network design special traffic management and concurrent users in each branch.

Flex VPN is a new framework to configure IPsec VPN with IKE version 2 (IKEv2) on IOS platforms.

Dynamic Multipoint VPN (DMVPN) is a Cisco IOS Software solution for building scalable IPsec Virtual Private Networks (VPNs).

Cisco offers three major routers types:

1. ISR: (integrated Service Routers):  typically used as internet edge for customers with small/medium networks.

2. ASR (Aggregation Services Routers): typically used as internet edge for enterprise networks and service providers.

3. CSR (Carrier Routing System): typically used in (IP/MPLS) core networks and for high-speed peering in smaller point-of-presence (POP) locations.

ASR is best choose but cost will be so important .

ASR routers are best suited for core network and ISR for boundary sites